A radioimmunoassay for rhesus monkey chorionic gonadotropin (mCG) employing an antiovine LH antiserum, 125I or 131I-ovine LH tracer, and mCG for standards was developed. Radioimmunoassay of serum levels of mCG during pregnancy indicated that mCG began increasing as early as day 12 after mating, reached a peak by day 25, and declined to nondetectable levels around day 35. The assay procedure was adapted for use as a rapid method for pregnancy diagnosis; the results were available as early as 12 hours after collection of serum samples. The method is capable of detecting a few pregnancies by day 12 and all pregnancies by day 17. Routine use of this method provided accurate pregnancy diagnosis four days earlier than was possible with the mouse uterine weight bioassay method previously used in this laboratory.