We demonstrate that peritoneal B cells have a much higher ability to present antigen to antigen-specific T cell lines than splenic B cells. Presentation of antigen by B cells is abrogated or drastically reduced after removal of Lyb-5+ cells from the population of splenic or peritoneal B cells. Peritoneal B cells, precultured for 7 days prior to the antigen presentation assay, retain their antigen presenting cell (APC) function. Enrichment for CD5+ cells in the peritoneal B cell population results in a more effective antigen presentation. Lastly, stimulation of B cells via CD5 antigen, by treatment of cells with anti-CD5 antibodies or cross-linking of CD5 receptors, enhances APC function of these cells. The results indicate, both indirectly and directly, that CD5+ B cells play a predominant role in the presentation of conventional antigens to antigen-specific T cells.