Ureaplasma species were isolated from semen samples collected sequentially from one Awassi and three Assaf breeding rams. Each ram was injected subcutaneously with an aqueous solution of lincomycin and spectinomycin for five consecutive days at a dose equivalent to 4.5 mg kg-1 lincomycin and 9.0 mg kg-1 spectinomycin daily. Serum and semen samples were collected at intervals during the treatment and assayed for lincomycin. No Ureaplasma species were isolated from semen samples collected during the course of the treatment and at intervals for 17 days after the last treatment. The concentration of lincomycin in semen ranged from 0.51 microgram ml-1 four hours after treatment to 0.08 microgram ml-1 24 hours after treatment, and these levels were three to nine times higher than the corresponding serum concentrations.