This study compares the effects of passive administration of monoclonal anti-hapten (DNP) antibodies on primary plaque-forming cell (PFC) responses in mice to either soluble (DNP-keyhole limpet haemocyanin [KLH] ) or particulate (TNP-erythrocyte) antigens. IgM, IgG1, IgG2a and IgG2b antibodies at doses up to 500 micrograms induced at best a modest suppression of the IgM response, and reproducibly enhanced the IgG response to DNP-KLH by up to 30-fold. In contrast, with the particulate antigen only the IgM antibody enhanced IgG PFC; IgG2 antibodies, and one out of two IgG1 antibodies caused marked suppression of the primary response to TNP-RBC. This required antibody with an intact Fc portion. The enhancement of IgG responses to soluble antigen presumably reflects rapid B cell priming by immune complexes trapped by follicular dendritic cells in lymphoid follicles, in agreement with earlier data. These results indicate that the nature of the antigen can markedly influence the immunoregulatory effects of antibodies on humoral responses.