The capacity of various bacterial components to induce antibody formation in human lymphocyte cultures was studied in the present investigation. Antibody levels were determined by an enzyme-linked immunosorbent assay (ELISA). Lipopolysaccharide (LPS), bacterial cell walls (CW, isolated from Bacillus subtilis and Staphylococcus aureus Wood 46) and peptidoglycans (PG) appeared to stimulate IgM, IgG and IgA secretion, whereas lysozyme-solubilized PG and teichoic acids (TA) were ineffective. Also, umbilical cord blood lymphocytes produced IgM after stimulation with LPS, CW and PG. Coculture experiments with purified lymphocytes and monocytes indicated that B-cell differentiation was dependent on both T cells and monocytes, and that T-cell derived factors could derived factors could partially substitute for T cells.