Bovine Concanavalin A-induced suppressor cells were generated from lymphocytes which were non-adherent to anti-immunoglobulin coated dishes and cells possessing receptors for peanut agglutinin. Bovine lymphocytes, preincubated with 25 microgram/ml of Con A for 40-45 hr, could suppress the responses of autologous cells to the mitogens Con A, PHA and PWM as much as 90% when they were cultured together at a ratio of 1:1 (suppressor cell to responder cell) or higher. Suppressor cells were not necessary at the initiation of the mitogenic assay as they could regulate responding cells if added at 48 hr in a 72 hr assay. Allogeneic responder cells could be suppressed at the same level as autologous cells indicating a lack of genetic restriction. Macrophages were not required for suppressor cell generation because peripheral blood lymphocytes (PBL) depleted of macrophages by Sepharose G-10 columns, and subsequently incubated with Con A, could suppress autologous cells to a similar degree as unseparated PBl's. Responder cells depleted of macrophages had normal mitogen responsiveness and were suppressed indicating macrophages were not required in transmission of suppressor signals. Cell to cell contact was not required for suppression connoting a soluble factor(s) as the modulator of suppression.