It is well known that the transfer of immunoglobulins (Igs) from mother to young via milk contributes to the offspring's immune defense. The present study suggests that not only is IgG transmitted to progeny, but that functional maternal Ig-secreting cells (or B cells) can also be transferred to the neonate. We have used B cell-deficient (μ^−/−) mice and found that a high proportion of them obtain long-lasting, partial reconstitution of their serum Ig levels if born to μ ^/− mothers. In some of these serum IgG-positive μ^−/− mice, Ig-secreting cells were detected in spleen and bone marrow. To ensure that cells of maternal origin were present in the progeny, μ^−/− offspring born to μ ^/− dams transgenic for green fluorescent protein (GFP) were used. In spleens and bone marrow from some of these μ^−/−GFP^−/− offspring, GFP-positive cells were detected, which demonstrated that cells of maternal origin could infiltrate the progeny. In addition, splenic Ig-secreting cells were detected in μ^−/− mice that were born to μ^−/− dams and transferred to a lactating μ ^/ foster dam at birth. This indicates that maternal Ig-secreting cells can be transferred postnatally via milk.