The aim of this study was to examine the maternal immune response to paternal antigens expressed by the fetus and identify the antigen inducing the response. Sera removed from responder female mice were tested for activity against paternal target cells using a cellular ELISA. Avtivity was first detectable at day 10 of a first pregnancy. The antibody detected in this way was shown to be non-cytotoxic, consisting of the IgGl subclass, directed against a class I antigen that could not be found on target erythrocytes. Sera removed at different stages of pregnancy exhibited varying degrees of cross-reactivity. To provide a source of pregnancy-induced antibody spleens from mice were removed during pregnancy and fused with rat or mouse myelomas. Antibody-secreting hybridomas were sought by means of CELISA with paternal cells as targets. Four hybridomas were isolated, producing antibody of the IgGl subclass, directed against a class I antigen and with limited cross-reactivity. The target antigen for both pregnancy sea and monoclonal antibody was examined for H-2 linkage, using the Lod score. The results obtained were unusual. Combination of the scores for four separate sera suggested an MHC-linked target. Individual scores suggested that two sera were directed against a linked and two against an unlinked antigen. Three of the monoclonal antidbodies were directed against H-2-linked antigens. Both sera and monoclonal antibody were immunoblotted against paternal, maternal and control cells. Pregnancy sera was seen to blot a 45-kD antigen present on paternal strain cells and cells from a mouse sharing the maternal haplotype. Only one hybridoma could be successfully blotted, revealing a 45-kD target. Immunisation with third-party lymphocytes has been used to treat recurrent spontaneous abortion. In twenty two couples treated in this way immunisation proved to be beneficial but there was no evidence for importance of an immune response or HLA sharing.