Three aminopeptidases (I--III) were purified from maternal serum using sequential chromatographic fractionations. Aminopeptidase I was specific for N-terminal alpha-L-dicarboxylic acid residues and activated by alkaline earth metals (Ba2+, Ca2+, Sr2+). It is concluded that aminopeptidase I is aminopeptidase A (L-alpha-aspartyl-(L-alpha-glutamyl)-peptide hydrolase, EC 3.4.11.7). Aminopeptidase II hydrolysed all tested substrates including L-cystine and Bz-L-cysteine derivatives but preferred L-leucine derivatives. The properties of aminopeptidase II are equal to those described for the cystine aminopeptidase (oxytocinase) (EC 3.4.11.3.). Aminopeptidase III preferred L-alanine derivatives as substrates. It was activated by Co2+, but strongly inhibited by amastatin, puromycin and L-methionine. The characteristics are reminiscent of those of alanine aminopeptidase (EC 3.4.11.-).