Experimental data confirming our earlier suggestion, that cholerae toxin (CT) possesses superantigen (SA) properties are presented. When used in very small doses, CT has been found to induce polyclonal activation of T lymphocytes, essentially exceeding that observed in classical T mitogens characteristic of SA. CT, in contrast to mitogens and similarly to other SA, is shown to display this activity only in the presence of antigen-presenting cells. Experiments with the use of monoclonal antibodies to the variable region of the beta-chain of the T-cell receptor (V beta TCR) have demonstrated that CT, similarly to other SA, are capable of inducing expression of certain types of V beta TCR and causing polyclonal activation of T lymphocytes carrying these types of V beta TCR. The presence of these properties gives grounds for regarding CT as SA. The SA activity of CT has been found to be linked with its subunit A.