The effects of rolitetracycline (50 micrograms/g i.v.) and testosterone proprionate (4 X 1 micrograms/g s.c.) singly and combined on liver weight, serum GPT, SDH and bromsulfophthalein clearance have been measured in young mice. Pretreatment with testosterone tended to enhance the effects of rolitetracycline on the serum enzymes but was otherwise not effective.