Monospecific antisera were prepared against the most prominent arylamidase (alpha-aminoacyl-peptide hydrolase (microsomal), EC 3.4.11.2) active antigen in plasma membranes (the plasma membrane arylamidase) and lysomal content (the lysosomal content arylamidase), respectively. Plasma membrane extract and lysosomal content were allowed to react in crossed immunoelectrophoresis against their homologous antisera. The electrophoretic plates were washed extensively, dried and subsequently stained for arylamidase activity. The particular immunoprecipitates were thus identified and could be excised to be used for immunizations. The two resulting antisera precipitated the arylamidase used for immunization, but failed to be monospecific as they precipitated additional antigens. These antisera with restricted specificity against some plasma membrane and lysosomal content antigens, respectively, were used to produce immunoprecipitates intended for new attempts to prepare monospecific antisera by a second cycle of immunizations. A monospecific antiserum against the plasma membrane arylamidase was thus obtained, while a third cycle of immunizations was needed to get a monospecific anti-lysosomal content antiserum. The plasma membrane arylamidase showed ATPase activity also after precipitation with the monospecific antiserum, thus still retaining its characteristics as a multienzyme complex.