Twenty-seven 2- to 4-yr-old cebus monkeys (Cebus albifrons) were fed from infancy purified diets containing lactalbumin, soy isolate, casein or soy concentrate as the sole protein source. Hematologic and clinical chemistry values were similar for all groups. Head and tail portions of each pancreas were surgically removed for histopathologic evaluation and determination of protein, RNA and DNA content, and for trypsin and chymotrypsin activity. Hematoxylin and eosin-stained sections from 26 of 27 monkeys showed normal pancreatic tissue with occasional acinar vacuolation in all diet groups. The remaining animal, one of only two fed soy concentrate, had diffuse interstitial fibrosis of the pancreas associated with mild to moderate atrophy of acinar tissue. Biochemical analyses of the pancreatic biopsies indicated no group differences among animals fed lactalbumin, soy isolate or casein. One of two monkeys in the soy concentrate group showed decreased pancreatic protein, RNA and trypsin concentrations; this was probably due to the fibrosis in this animal. No evidence of pancreatic hypertrophy or hyperplasia, as measured by RNA/DNA and protein/DNA ratios, respectively, was seen in any diet group.