We describe the development of radioimmunoassays to measure both human pepsinogen A and pepsinogen C concentrations in serum. The antibodies were raised in goats by immunization with purified pepsinogen A or C. The affinity constants of the respective antibodies were 20.10(10) l/mol and 7.10(10) l/mol. Pepsinogens A and C were labeled with Na 125I by the chloramine T method. The binding between labels and antibodies was inhibited by 0.50 at 0.82 ng pepsinogen A per tube and 2.1 ng pepsinogen C per tube. The detection limits of the assay of pepsinogen A and C were 0.12 microgram/l and 1.8 micrograms/l, respectively. Pepsinogen A and C were purified and added to a patient serum, showing a good recovery in the radioimmunoassays. Serial dilution of another patient serum, which contained a high concentration of both antigens, showed curves parallel to the standard curves. The intra- and interassay variations of these radioimmunoassays were evaluated. The intra-assay coefficients of variation for pepsinogen A were found to vary from 0.03 to 0.102 at concentrations in serum in the normal range, while the inter-assay coefficient of variation ranged from 0.118 to 0.194 at the same concentrations in serum. For the pepsinogen C radioimmunoassay we found intra-assay coefficients of variation between 0.126 and 0.147 at concentrations in serum in the normal range, while the inter-assay coefficient of variation ranged from 0.174 to 0.325 for the same sera. In 201 blood donors we found a mean serum concentration of pepsinogen A of 59 micrograms/l and a mean serum concentration of pepsinogen C of 15 micrograms/l. There was a significant relationship between these values (r = 0.779, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)