Immunoglobulin (Ig) production by lipopolysaccharide (LPS)-stimulated cells in the presence of various combinations of interleukin (IL)2, IL4 and IL5 was examined. IgG1, IgM and IgE secretion was studied using a 3T3-fibroblast filler cell-supported B cell culture system, either at low cell density to support maximal Ig secretion, or at limiting dilution to determine isotype-specific precursor frequencies. In the presence of optimal concentrations of IL5 (2%) and IL2 (3 U/ml), the addition of 1 U/ml of IL4 resulted in the production of 4 ng of IgG1 per input B cell. In contrast, 1000 U/ml of IL4 alone was required to produce equivalent levels of IgG1. IL5 and IL2 increased both the precursor frequency and the amount of IgG1 secreted per clone in the presence of low levels of IL4. On the other hand, IgM secretion was decreased 10-fold by the addition of 10 U/ml IL4 or greater. This was not seen when IL5 was present. The IgM-secreting precursor frequency was unaffected by any of the lymphokines, either singly or in combination. The inhibition of IgM production and subsequent relief of this with IL5 was shown to affect the amount of IgM secreted per clone. IgE secretion was shown to be highly IL4 dependent with only minor reduction in the required concentration following addition of IL5 and IL2. At the clonal level, the majority of IgE-secreting clones (93%) at high IL4 concentrations (200 U/ml) arose from precursors which were able to produce IgM and IgG1. Furthermore, only 3% of the clones secreted IgG1 alone, with a further 3% secreting IgE alone. These results suggest that B cells in vivo are predominantly uncommitted in terms of isotype to be produced, the choice of isotype secreted being dependent on the nature of the stimulus. Overall, this work shows that the isotype secreted by B cells can be regulated using combinations of IL2, IL4 and IL5, and that major effects can be achieved by very small quantities of lymphokines acting in synergy.