Feeding soy-based protein containing trypsin inhibitor causes pancreatic hypertrophy in the rat, and long-term feeding (up to 2 years) has revealed a high incidence of adenoma following hypertrophy. It was therefore of interest to determine whether the ingestion of soy-based protein has any adverse effects on the primate pancreas. A resource of 27 Cebus albifrons monkeys, previously used to evaluate the protein quality of several soy and milk proteins, has been maintained on semi-synthetic diets for 3 to 4 years; the protein sources for the diets were casein, lactalbumin, soy isolate and soy concentrate. In general the monkeys were in good physical health and their weights were appropriate for age and sex. Serum biochemical and hematological profiles were normal and there were no major differences between the groups. A pancreatic biopsy from both the head and tail region of the pancreas was taken from each monkey. Visual observation of the pancreas revealed no overt pathology; two independent histological examinations indicated no diet-related differences between groups, and biochemical analyses of trypsin, chymotrypsin, protein, DNA and RNA revealed no differences. It is concluded that feeding low level trypsin inhibitor-containing diets for up to 4 years caused no adverse effects in the pancreas of the Cebus nonhuman primate.