The addition of ovalbumin-immune spleen cells (Ova ISC) to sheep erythrocytes-immune spleen cells (SRBC ISC) in Mishell-Dutton-type cultures resulted in a dramatic reduction of PFC to SRBC and was dependent upon the addition of soluble Ova at low concentrations of Ova ISC. The suppressing cells in the Ova ISC were shown to be irradiation sensitive, depleted by anti-theta antiserum and complement treatment, and did not absorb to glass bead columns. Ova ISC-induced inhibition also occurred in culture chambers across a cell-impermeable membrane and a soluble inhibitor was recovered in chambers opposite the Ova ISC. This suppressor factor was sensitive to trypsin treatment and to heating at 80 degrees C, but not to 70 degrees C, for 30 min. The molecular weight, as determined by sucrose gradient analysis, was between 55,000 and 60,000 daltons. This suppressor factor appears to be distinct from a T-cell "helper" factor which was found to be sensitive to heating at 70 degrees C for 30 min. We propose that this suppressor factor participates in the termination of most immunologic responses and is responsible for the antigenic competition phenomenon.