The aims of this study were to produce and characterize mouse monoclonal antiidiotypic antibodies (AIAs) that specifically bind human antipig (antiαGal) antibodies and to select those AIAs that neutralize the cytotoxicity of human or baboon serum to pig (PK15) cells. Mice were immunized with human antipig antibodies, and hybridomas were produced using conventional techniques. From a total of 480 clones, 11 produced AlAs that bound with high affinity to human anti-αGal IgG and F(ab')2 fragments, and individually reduced serum cytotoxicity to pig cells by 40 - 90%. Seven of the AlAs also bound to human peripheral B lymphocytes (that express the same idiotypes as the antibodies they produce). Several combinations of two or three AIAs provided 100% protection of PK15 cells. Selected AlAs injected intravenously into baboons reduced the cytotoxicity of subsequently drawn sera by 50-80% for >48 hr. The cytotoxicity studies also indicated that there are at least two dominant idiotypes expressed in the human anti-αGal population. We conclude that AlAs against anti-αGal antibodies could be successfully used in (1) preoperative characterization of a recipient's anti- αGal profile, (2) prevention of hyperacute rejection of a pig organ, and (3) specific immunosuppression through elimination of anti-αGal-producing B lymphocytes.