<Page>

                                                                      Exhibit 21

                            GTC BIOTHERAPEUTICS, INC.

                           SUBSIDIARIES OF THE COMPANY


ATIII LLC, a Delaware limited liability company

GTC Securities Corporation, a Massachusetts corporation

GTC Cancer Vaccines, Inc., a Delaware corporation

GTC Japan Limited, a company organized under the laws of Japan

GTC Holding, Ltd., a company organized under the laws of the Cayman Islands

Taurus hSA LLC, a Delaware limited liability company

TSI Corporation, a Delaware corporation

Transgenic Investments, Inc., a Delaware corporation

TSI Deutschland GmbH, a company organized under the laws of Germany