A simple, rapid, highly sensitive and reproducible fluorimetric assay for angiotensin-converting enzyme in untreated serum using the natural substrate based on a similar assay with hippuryl-L-histidine-L-leucine is described. Angiotensin I (0.2 mM in 0.1 M potassium phosphate-30 mM NaCl, pH 7.5; 37 C) is converted to angiotensin II and L-histidyl-L-leucine, which is quantified fluorimetrically (excitation = 360 nm; fluorescence, 500 nm) by formation of a fluorescent adduct with O-phthaldialdehyde. L-Histidyl-L-leucine peptidase was also monitored in order to correct for significant activity, which was observed only once, in less than 1% sera. The mean value of serum angiotensin-converting enzyme in sera from 45 normal subjects was 4.69+/-0.194 (SEM)+/-1.30 (SD) nmol/min/ml serum, compared with 31.7+/-1.53 (SEM)+/-10.3(SD) with the substrate analog hippuryl-L-histidine-L-leucine. There was a high degree of correlation between the velocity of cleavage of angiotensin I and hippuryl-L-histidyl-L-leucine (r - 0.903 to 0.993). The assay of serum angiotensin-converting enzyme is of use in the diagnosis and possible management of sarcoidosis and Gaucher's disease, and may have other applications.