This study addresses itself to the problem of antibody formation in vitro by mouse splenic B lymphocytes enriched for reactivity to the hapten NIP by the hapten-gelatine binding and melting technique of Haas and Layton (1975). Small numbers of NIP-gelatine-bound B cells were placed in microcultures either by bulk dispensing of dilute cell suspensions, or by micromanipulation under direct microscopic visualization. Antibody formation was induced by the T cell-independent hapten-protein conjugate NIP-polymierized flagellin, using 10(4) thymus cells per microlitre as 'filler' cells. The frequency of precursors of NIP-specific antibody-forming cells among bound cells was about 2-2 X 10(-2) (one cell in forty-five) by both statistical and direct evaluation, after adjustment for a background frequency of 6-10 X 10(-8) precursors in the thymus filler cells. Single clones commenced antibody secretion asynchronously, as shown by the fact that the incidence of positive cultures continued to rise over the whole three days of culture, and that very small clones of one to four plaque-forming cells (PFC) were still found on day 3. The mean PFC number per positive culture rose from 1-2 at day 1 to 4-7 at day 2 and about 20 at day 3.