Mice of the H-2b haplotype were mated with males of the same MHC haplotype, but differing at multiple minor histocompatibility loci. Mice were bled during each pregnancy and at 2-day intervals post-partum. The sera were assayed by indirect immunofluorescence for evidence of a humoral immune response to paternal minor histocompatibility antigens. Alloantibody was first detected in the post-partum period following the third pregnancy, and was also detected during the fourth pregnancy. Thereafter, alloantibody levels dropped and by the post-partum period following the fifth pregnancy, fell to control values. Assays on a panel of cells from mice of different inbred strains revealed specificity of the alloantibody to H-3.1, H-4.1 and H-7.1 antigens. A conventional dye exclusion cytotoxicity test revealed the pregnancy-induced alloantibody did not exhibit complement-dependent cytotoxicity. These findings are discussed in relation to the regulation and functional significance of the humoral immune response in allogeneic pregnancy.