A good animal model that simulates the human subject has not been available for the evaluation of the in vivo effectiveness of vaginal contraceptives. After careful consideration, The stumptailed macaque (Macaca arctoides) was studied for its applicability since it has a reproductive tract similar to that of the woman, is easy to handle, does not require tranquilization or anesthesia when the contraceptive is deposited, and breeds and conceives readily under caged conditions. The reported observations show the usefulness of this animal. Both postcoital sperm motility studies and breeding experiments were performed with the use of Delfen vaginal cream and K-Y jelly. K-Y jelly had no effect on the motility of vaginal spermatozoa or on the conception rate of the primates. Although Delfen vaginal cream consistently immobilized all spermatozoa in the postcoital test, half of the animals became pregnant within an average of 3.7 breeding cycles. These results illustrate the discrepancy between spermicidal tests and fertility measurements, and it is recommended that primate-breeding experiments be performed before a spermicide is evaluated in women as a contraceptive. (AM J. OBSTET. GYNECOL, 129:368, 1977.)