The electrophoretic mobilities of proteases present in gastric juice taken within 10 h of birth from 5 healthy, premature infants were compared with calf chymosin, pig pepsin A and human adult gastric juice. The juice from 2 infants contained predominantly a chymosin-like enzyme, another had almost exclusively pepsins similar to those of the adult juice, while the other two contained a mixture of both. The pepsins consisted of two elements, probably pepsin A (EC 3.4.23.1), and pepsin C (EC 3.4.23.3). Single radial immunodiffusion gave a definite reaction to calf anti-chymosin serum in five samples taken from a further 17 infants. These results indicate that some human infants secrete chymosin. The reaction in the immunodiffusion assay indicated a much lower enzyme activity than that implied from electrophoretic separations. It is suggested that species differences resulted in poor cross-reactivity of the antiserum.