Cultures of mouse fetal liver and spleen, stimulated by bacterial lipopolysaccharide, gave rise to plasma cells staining for IgM, IgG2, IgG1, and IgA. Cells containing Igm and IgG2 were found in cultures from 17-day fetuses, coincident with the appearance of B lymphocytes bearing cell-surface IgM. IgG1- and IgA-containing cells were induced in cultures from 19-day fetuses and 1-day-old mice. The capacity to give rise to immunoglobulin-secreting cells of all classes preceded the development of a significant proliferative response to LPS; the proportions of cells staining for each class reached adult values by 1 day of age whereas the proliferative response did not mature until 3 weeks.