The preparation of monospecific antisera for immunoassays of human placental (P-AP, Regan) and intestinal (I-AP) isoenzymes necessitates complete absorption of the antisera with heterologous antigen. Large scale preparation of pure human I-AP for absorption of anti-P-AP sera is not practical since a large amount of fresh intestine is required. Calf I-AP cross-reacts with human I-AP, but evidence is presented that this antigen is not suitable for absorption of anti-human-P-AP sera. In contrast, human I-AP can be extracted from faeces of patients under polychemotherapeutic treatment and used for the complete absorption of anti-human-P-AP sera. Monospecific anti-human-I-AP sera, suitable for immunofluorescence studies, can be obtained by a two-step affinity chromatography procedure: (1) human I-AP is removed from an extract of small intestine by antibodies to cross-reacting calf-I-AP, and (2) this small intestinal mucosal extract, free of human I-AP, can be used for the absorption of anti-human-I-AP sera. Finally, we present evidence that antibodies to calf-I-AP precipitate human-I-AP and can be used in immunoassays for I-AP in man.