The effect of autologous normal mouse serum on activating properties of the polyclonal B cell activators lipopolysaccharide and purified protein derivative was studied. ‘Low’ concentrations (0.1–1%) were found to be slightly stimulatory, whereas ‘high’ concentrations (≥ 10%) were clearly inhibitory both for the induction of plaque-forming cells and for the DNA synthetic response. Similar effects were recorded in unstimulated cultures, Absorption of normal mouse serum with the erythrocytes used as target cells in the plaque assay resulted in less suppression or sometimes even in enhanced responses.