CBA mouse spleen cells were subjected to hapten affinity fractionation on thin layers of fluorescein (FLU)-gelatin. This procedure yields 97% B cells with varying FLU-binding avidities. One to 30 cells were placed in 10-microliter microcultures without any filler or accessory cells. the T-independent antigen polymerized flagellin coupled to FLU (FLU-POL) was ineffective in stimulating these cells to clonal proliferation or antibody production when used alone. Unpurified preparations rich in interleukins also failed to stimulate the cells. When specific antigen, but not in irrelevant hapten-POL, was combined with the interleukins, clonal proliferation was stimulated and most clones produced anti-FLU antibody-forming cells. The frequency of antibody-forming clones was only slightly lower than that in a system using antigen plus filler cells. In the absence of added interleukins, the mitogens Escherichia coli lipopolysaccharide plus dextran sulfate induced equivalent antibody production. However, a higher frequency of clonal proliferation was noted. Added interleukins did not aid these mitogen-driven responses. Such an antigen-dependent cloning system, free of filler and accessory cells, should permit more precise analysis of the respective roles of antigens and interleukins in the physiology of antibody-forming clone formation.