Consecutively collected semen samples from a breeding bull were found to be contaminated with Pseudomonas aeruginosa. Palpation through the bull's scrotum revealed inflammatory changes suggestive of chronic orchiepididymitis in one testicle. For 10 months, all the bull's 13 ejaculates were discarded because the post-thaw viability was < 20%. Norfloxacin nicotinate was injected intramuscularly into the bull at 5 mg/kg daily for 7 days. Serum and semen samples were collected at 24-h intervals during the course of treatment and afterwards and were assayed for NFN concentrations. Drug concentrations in the semen, by microbiological assay, during treatment and up to 120 h after the last treatment ranged from 2.6 to 5.1 micrograms/ml, 14.2 to 43.2 times the corresponding serum drug levels. P. aeruginosa was not isolated from the semen 4 or 15 days after the last injection but was re-isolated after 32 and 64 days. A second similar course of NFN was administered and P. aeruginosa was not isolated from semen samples collected on four occasions, 6, 22, 44 and 94 days after the last treatment.