Culture supernatants from concanavalin-A (con-A)-activated peripheral blood lymphocytes from healthy controls grown in the presence of sera from 20 patients 24 hours and 1 week after acute myocardial infarction (AMI) were tested for their mitogenic activity and for the presence of interleukin-2 (IL-2). Binding of exogenous IL-2 to activated lymphocytes from 10 patients was also determined. In supernatants prepared in the presence of patients' as compared to control sera, a significantly decreased mitogenic activity and IL-2 content were found. The mitogenic activity and IL-2 content in culture supernatants prepared with patients' sera collected 24 hours after the AMI (AMI I) and one week thereafter (AMI II) were significantly suppressed, and the degree of suppression in the 24-hour sera was significantly higher than in those collected after one week. No significant differences were observed in the binding capacity to exogenous IL-2 of activated patients' and control lymphocytes. The possibility is that immunosuppressive factors in the patients' sera, including cortisol, may suppress the patients' immune response acting through inhibition of IL-2 production.