IgE-bearing lymphocytes were detected by immunofluorescence in the spleen of neonatal Hooded Lister strain rats within 24 hr after birth. The same cells were detected in the bone marrow as early as the 4th day after birth. Both fetal spleen and liver obtained 1 day before birth contained IgM-bearing cells but no detectable IgE-bearing cells. The proportion of IgE-bearing cells in the spleen and bone marrow increased during the neonatal period and reached an adult level within 3 to 4 weeks after birth. In adult Hooded Lister rats, IgE-bearing cells were 3 to 6% of total spleen cells and 1.5 to 2.2% of bone marrow cells. Most of the IgE-bearing cells from both newborn and adult animals carried IgM determinants on their surface. Capping experiments showed that µ chain determinants and µ chain determinants belonged to separate molecules. IgG2a-bearing lymphocytes were detected in the neonatal spleen as early as the 4th day after birth, but a significant number of these cells was not detected in the bone marrow until the 4th week. In newborn spleen the percentage of IgE-IgM double bearing cells was higher than that of IgG2a-bearing cells.