Monoclonal antibodies (MABS) were raised against human alpha[unk]acid glycoprotein (AGP) and their reaction with the polymorphic forms of this plasma protein were evaluated. Spleen cells of BALB/c mice, immunized with native or desialylated human AGP, were fused with NSO mouse myeloma cells. The hybridoma products were screened with a direct ELISA test, in which the immunoplates were coated with a mixture of native and desialylated AGP. In this test, 14 anti - AGP antibody producing clones were retained. Coating the wells with either native or desialylated AGP showed that eleven clones reacted with both types of AGP ( ' Type I' MABS), while three MABS reacted specifically with the desialylated form ( ' Type II' MABS). Precoating the immunoplates with polyclonal anti-human AGP followed by incubation with native or desialylated AGP before the addition of hybridoma supernatant ( indirect ELISA), confirmed the specificities observed in the direct ELISA. The molecular heterogeneity of both native AGP and desialylated AGP, based on Concanavalin A reactivity and isoelectric point, was not reflected by any specificity in the antibody reactions. Thus ' Type I' MABS reacted with all molecular forms present in native and desialylated AGP while ' Type II' MABS reacted with all molecular forms present in desialylated AGP.