Bispecific antibodies (BAb) direct T-lymphocytes to lyse selected tumour targets, both in vitro and in vivo. Significant tumour cell lysis with BAb requires pre-expansion of T-lymphocytes, which may be achieved in vitro by the addition of anti-CD3 monoclonal antibody plus interleukin-2 (IL-2), but anti-CD3 may cause immunosuppression. We investigated an alternative agent for in vivo immunostimulation, staphyloccal enterotoxin B (SEB), which selectively activates certain T-cell subsets and may result in less immunosuppression than with anti-CD3. We activated T-lymphocytes in vivo with SEB, expanded them in vitro with IL-2, and directed them against a tumour target with the BAb 500A2 x 96.5, specific for the murine CD3 antigen and the melanoma p97 antigen expressed by the CL62 tumour. C3H mice received SEB 50 micrograms intraperitoneally (i.p.). After 18 h mice were sacrificed and splenocytes extracted and either passed over a nylon wool column to isolate T-lymphocytes, or cultured in vitro for 3 to 7 days with 100 U ml-1 of IL-2. A 4-h chromium-release assay was used to assess the ability of T-lymphocytes to lyse the tumour target CL 62 in the presence or absence of the bispecific antibody 500A2 x 96.5. The addition of BAb significantly enhanced tumour lysis by SEB activated cells after a period of in vitro culture with IL-2. In vivo SEB results in the activation of T-lymphocytes which may be directed by bispecific antibodies to increase the lysis of selected tumour targets in vitro.