Many bursa-equivalent (B) lymphocytes of adult mice bear surface Ig and receptors for C3. The frequency of Ig-bearing cells increases rapidly immediately after birth, but these cells lack complement (C) receptors. Lymphocytes bearing C receptors are not found in the spleens of BALB/c, DBA/2, and C57BL/6 mice until 2 wk of age and do not attain substantial numbers until 3-4 wk of age. In AKR mice, a lag between appearance of Ig-bearing and complement receptor lymphocytes (CRL) is also observed but it is of much shorter duration. AKR mice have a frequency of CRL at 2 wk of age of 28% in comparison to a frequency of 4.8% for DBA/2 mice. The difference in frequency between young and adult mice and between "low" and "high CRL" strains cannot be explained by a nonspecific inability to form rosettes as similar results are obtained with soluble antigen-antibody-complement complexes. Analysis of CRL frequency in (AKR x DBA/2)F(1) mice and F(1) x parental backcross progeny suggests that two independent genes control the rate of appearance of CRL. Furthermore, the genetic difference in the ontogeny of CRL is recapitulated in the repopulation of the B-lymphocyte line in adult-irradiated mice restored with syngeneic bone marrow. Thus, the "CRL genes" described here appear to control B-cell differentiation throughout life.