Transmission of immunoglobulin (Ig) classes and subclasses from mother to foetus and to neonate, and the survival of maternal Ig in the circulation of the young mouse up to 40 days after birth, has been quantitated in Balb/c homozygous and (Balb/c X SJL/J)F1 matings using isotype-specific heteroantisera in radial immunodiffusion in gel assays. The transfer of anti-allotype (anti-Ig-1b (gamma 2a] antibodies from immunised Balb/c mothers (Ig-1a) to F1 heterozygote (Ig-1ab) offspring was measured by passive haemagglutination of Ig-1b target allotype-coated sheep red blood cells. A small but significant level of transmission of Ig to the foetus occurs by the 15th day of gestation (5 days before birth) but the bulk of passively acquired Ig is derived from the milk after birth. All Ig acquired in utero and later across the intestinal barrier is exclusively of IgG isotypes (gamma 1, gamma 2a, gamma 2b) even though the milk has a large predominance of IgA. An appreciable level of maternally derived antibody is maintained in the circulation of the young mouse 24 days or more after gut 'closure' on the 16th day post-partum.