Cultured bone marrow cells, after in vitro treatment with hydroxyurea (HU) - a DNA synthesis inhibitor which kills cells in the S phase of the cell cycle - generated 40 to 70% more B cells than untreated control cells. This was shown by fluorescent-activated cell sorter analysis of labelled cells using FITC-F(ab')2 rabbit anti-mouse IgM and functional tests with LPS. The maximum increase was reached after 24 hr of incubation with HU while 6 or 2 hr of exposure had less effect. The effect of HU was dose dependent with a maximum at 4 mM. The same increase of B cells was observed with foetal liver cells but not with spleen or lymph node cells after 24 hr of in vitro HU treatment. Dialysed supernatants from HU treated bone marrow, spleen or foetal liver cells were themselves able to augment the B cell maturation in bone marrow cultures (test cells) as compared with supernatants from untreated cells, showing that soluble factors were involved. Preliminary data showed that inhibitory factors for B cell maturation were produced by normal bone marrow, spleen and thymus cells in vitro and their formation was prevented by HU pretreatment or irradiation (2500 R) whereas stimulatory factors were produced by lymph node cells. Cell separation experiments suggested that T cells and/or adherent cells may be involved in the production of these soluble factors. These data suggest that early B cell development may be under homeostatic control.