The ontogeny of Ig [immunoglobulin], FcR [Fc receptor], and CR[complement receptor]-bearing cells in liver and spleen was followed by using rosetting procedures. These studies demonstrated a sequential appearance of surface receptors during development. Two types of Ig+ cells could be distinguished according to their rosette morphology and adherence to carbonyl Fe: an adherent cell which bound few erythrocytes was found predominantly in fetal liver from 13 days gestation; and a nonadherent cell, which could bind larger numbers of erythrocytes, appeared in small numbers in fetal liver from day-16 gestation but represented the major Ig+ cell type after birth. Changes in the proportions of receptor-bearing populations occurred at 2 particular periods during ontogeny. The first was at birth, where an increase in the proportion of FcR+ cells occurred and the proportion of type 2 Ig+ cells rose rapidly. This probably represented the 1st appearance of FcR+ B [bone marrow-derived] lymphocytes, even though cells bearing FcR were detected in fetal liver of all ages (days 12-18). The 2nd period was about 10 days after birth when the proportion of Ig+ cells again increased concomitant with the appearance of CR+ nonadherent cells.