We used immunofluorescence to examine the developmental relationship of Ia and IgD on B cells. Pre-B cells in fetal liver did not express Ia. Only very few surface IgM-positive (sIgM+) B cells in fetal spleen were found to be Ia+ and were weakly stained for Ia. After birth there was a linear increase in the proportion of sIgM+ spleen cells which expressed Ia, reaching 95% by 9 days. Adult bone marrow also contains a sizeable proportion of sIgM+ Ia- cells. Unstimulated cells from fetal or newborn liver and spleen expressed Ia at the same rate in culture. Anti-Ia antisera suppressed the LPS-induced differentiation of IgM and IgG plasma cells in cultures of neonatal lymphocytes. Ia was also detected on IgM and IgG plasma cells in vitro suggesting that lipopolysaccharide (LPS)-stimulated B cells by may express Ia antigens, induced by LPS, or appearing as part of normal differentiation. IgD did not appear on sIgM+ cells until 3 days of age and then rose slowly to reach adult levels later than Ia antigens.