A selection process has been developed by the authors to insure investigators that animal subjects used in their experiments will accurately reflect biological changes associated with the project design. The procedure involved the following: complete physical examination; complete hemogram, serum electrolytes, serum biochemistry, body weight; and anthropometric determinations; aging by dentition; electrocardiography; total-body radiography; fecal culture; fecal ova, and parasite examinations; and examination of other conditions. Arithmetic means and standard deviations were calculated and used to eliminate monkeys whose values varied from the mean by more than two standard deviations.