Human lymphocytes from different lymphoid organs were examined for rosette formation with autologous erythrocytes. The autorosette-forming cells (A-RFC) were shown to belong to a T cell subset including less mature lymphocytes. When normal human peripheral blood lymphocytes were stimulated with how doses of the plant lectin concanavalin A (Con A), in the presence of autologous plasma, the A-RFC levels were strongly enhanced. This response gave rise to two peaks: the first one coincided with the peak of thymidine incorporation but the maximum increase occurred 5 or 6 days later when the proliferative response was impaired. Depletion of A-RFC before stimulation with Con A led to a clearcut decrease in autorosette levels at both peaks of the response. It is concluded that Con A, generally used for polyclonal activation against heteroantigens, may also result, in terms of A-RFC marker, in expansion of an autoreactive T cell population.