Growth factor activity in luminal fluids from the male reproductive tract was assayed by measuring the stimulation of [3H]thymidine incorporation into BALB/c 3T3 fibroblasts. The potency of fluids from the rete testis of the rat, ram, tammar wallaby and Japanese quail was much the same. However, about 90% of the activity in fluid from the rete testis of the rat and tammar was lost during its passage through the epididymis.