Injection of mice already making antibodies to 2,4-dinitrophenylated (DNP) Ficoll with tolerizing doses of DNP-pneumococcal polysaccharide (DNP-lys-S3) markedly inhibits the secretion of anti-DNP antibodies by IgM antibody-forming cells. The present study shows that the degree of inhibition depends not only on the dose of DNP-lys-S3 but also on the duration of exposure to antigen. DNP-lys-S3 was detectable on the surface of antibody-forming cells at a time when their rate of secretion was unimpaired, thus suggesting that the inhibition involves intracellular events subsequent to the binding of antigen to the cell membrane. The inhibition was reversible if antibody-forming cells were exposed to antigen for 24 h, and then cultured for 18 h in its absence, but became irreversible if the treatment period was extended to 48 h. The relevance of this model of inhibition of lymphocyte function by antigen to possible mechanisms of B cell tolerance is discussed.