A study was made of 11 workers exposed to zeranol (55331298) powder during the manufacturing of the animal growth regulator at the manufacturing facility. The workers included five production and packaging personnel who had regular direct skin contact with the chemical during mixing and sifting of zeranol powder or during packaging of the finished pellets into plastic containers. The other six exposed workers were administrative, laboratory, and cleaning staff with intermittent contact. Exposure was via skin contamination, inhalation, and probably involuntary ingestion as well. An excess of breast symptoms was reported among the male and female workers, but the numbers were small and differences insignificant. No differences were noted regarding intermenstrual bleeding among the exposed and nonexposed women. There was also no impotence, loss of libido, or gynecomastia among the men. Serum luteinizing hormone and follicle stimulating hormone levels in exposed female workers were within normal limits. No differences were found in serum prolactin, estradiol, total cholesterol or in low density lipoprotein cholesterol levels between exposed and nonexposed groups.