HYBRIDON,  INC.  SUCCESSFULLY  EXCHANGES  $48.6  MILLION  OF DEBT FOR EQUITY AND
REACHES $27.3 MILLION IN 1998 PRIVATE PLACEMENT PROCEEDS

CAMBRIDGE,  Mass.,  May 6, 1998 -- Hybridon,  Inc.  (OTC-Bulletin  Board:  HYBN)
announced  today that it has closed a private  placement  of  approximately  6.6
million  shares of  Common  Stock  (sold at $2.00 per  share)  and  warrants  to
purchase common stock and  approximately  114,300 shares of Series A Convertible
Preferred  Stock (sold at $70 per share) and warrants to purchase  common stock.
Gross proceeds were  approximately  $21 million,  including  approximately  $6.7
million  which was applied to reduce  existing  payables  and satisfy  lease and
other obligations.  In addition,  Hybridon  announced that  approximately  $48.6
million principal amount of its 9% Notes have been tendered to the Company to be
exchanged  for shares of Series A  Convertible  Preferred  Stock and warrants to
purchase  common stock pursuant to the Company's  exchange  offer,  leaving only
approximately $1.4 million principal amount of 9% Notes  outstanding.  Moreover,
Hybridon  announced that all of the recent purchasers of its Units consisting of
14% Notes due 2007 and common stock  warrants have agreed to exchange such Units
for Common Stock  (priced at $2.00 per share) and  warrants,  bringing the total
proceeds of the offering to date to approximately $27.3 million.

Funding came from  investors in the United  States,  Europe and the Middle East,
and included the renewed participation of several large existing shareholders of
the Company, as well as significant holders of 9% Notes,  Hybridon creditors and
new  institutional  investors.  The private  placements  and exchange offer were
undertaken as part of Hybridon's new business plan contemplating a restructuring
of its capital  structure  to reduce debt  service  obligations,  a  significant
reduction in its burn rate and an infusion of additional equity capital.

The securities  offered in the private placements have not been registered under
the Securities Act of 1933, or applicable  state securities laws, and may not be
offered  or sold  absent  registration  under  the  Securities  Act of 1933  and
applicable state laws or available exemptions from registration.

The  statements  made in this  press  release  contain  certain  forward-looking
statements, including statements relating to the future financial performance of
the Company, within the meaning of Section 21E of the Securities Exchange Act of
1934, that involve a number of risks and uncertainties,  including the risk that
the Company will be unable to complete future  securities sales. Such statements
are only  predictions  and actual  events or results may differ  materially.  In
addition to the matters described in this press release,  risk factors as stated
from time to time in Hybridon's  SEC reports,  including but not limited to, its
Annual Report on Form 10-K, may affect the results achieved by Hybridon.

Hybridon, headquartered in Cambridge, Massachusetts, is engaged in the discovery
and  development  of novel  genetic  medicines for the treatment of diseases for
which there are currently limited or no effective treatments, based primarily on
antisense  technology.  The Company  also has a custom  manufacturing  division,
Hybridon Specialty Products.  Antisense technology involves the use of synthetic
segments  of DNA to stop  the  production  of  disease  associated  proteins  by
interacting at the genetic level with target strands of messenger RNA.

SOURCE Hybridon, Inc.