The ontogeny of immunoglobulin (Ig)-bearing and antigen-binding lymphocytes in guinea pigs was examined. At 54 days of gestation, fetal spleen contained adult percentages of IgM-bearing lymphocytes, but few which bore either IgG1 or IgG2, immunoglobulin. By term, the fraction of cells bearing each of the major Ig classes had reached, or was approaching, adult levels. In addition, the appearance of Ig-bearing cells with receptors specific for 2,4-dinitrophenyl (DNP) groups was determined. By 50 days of gestation considerable numbers of these cells were found. Virtually all these cells possessed surface Ig of the µ class. Only at term did IgG2-bearing DNP-binding cells appear. In the adult guinea pig, the majority of DNP-specific lymphocytes possess surface Ig of the IgG2 class.