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Cystine aminopeptidase and arylamidase activities in human serum were determined by enzymic hydrolysis of L-cystine-di-beta-naphthylamide (CysNA) and L-leucine-beta-naphthylamide (LeuNA), respectively. The activities of both enzymes increased during pregnancy, cystine aminopeptidase 12.5-fold and arylamidase 8.3-fold. Serum CysNA and LeuNA hydrolysing aminopeptidases were separated by gel filtration on Sepharose 6B. Serum from non-pregnant women (control) contained arylamidase (Ic), which hydrolysed LeuNA and (weakly) CysNA, and cystine aminopeptidase II, hydrolysing only CysNA. During pregnancy a new enzyme appeared in maternal serum and showed cystine aminopeptidase and arylamidase activity (Im). Maternal serum Enzyme(s) I had higher pH optima (6.5 with CysNA; 7.5 with LeuNA) and higher molecular weights (309,000) than arylamidase Ic (pH optima at 5.52-5.5 with CysNA and 7.0 with LeuN; mol.wt approximately 130,000). Arylamidase Ic was more sensitive to L-methionine, but more resistant to heat than maternal serum Enzyme(s) I. Both control and maternal serum Enzyme(s) I were inhibited by EDTA, but were re-activated by Zn2+ and Co2+ with LeuNA and CysNA as substrates and by Ni2+ with CysNA. Cystine aminopeptidase Im and arylamidase Im may be a single enzyme although differences were obtained in pH optima and reactivation by Ni2+ after EDTA treatment. Since maternal serum Enzyme(s) I had biochemical characteristics similar to those of placental aminopeptidase(s) I, it is suggested that the activities are of placental origin. Cystine aminopeptidase II appeared in all sera. It differed clearly from both maternal and control serum enzyme(s) I: it had the lowest molecular weight (76,000), a different pH optimum (6.0) and was resistant to EDTA and L-methionine. It was not as effectively inhibited by Ni2+ as was Enzyme(s) I.