Concentrations of erythromycin in maturing chinook salmon after intraperitoneal injection of one of two drug formulations

A dose (10 mg/kg) of erythromycin phosphate or erythromycin base was inject ed intraperitoneally into maturing spring chinook salmon Oncorhynchus tshaw ytscha on one of three dates before sexual maturity to observe if differenc es in the concentrations of erythromycin remaining in tissues 14 d after in jection could be attributed to the sex of fish, drug formulation, or timing of injection. No differences due to formulation of erythromycin or date of injection were detected in kidneys or spleens, but females injected with e rythromycin phosphate had significantly higher levels of erythromycin/mL in the maturing ova than did fish injected with erythromycin base. Mean conce ntration of erythromycin in kidneys of fish was 1.6 mu g erythromycin/g wet weight. Thirty-six of 37 females and 18 of 29 males had detectable erythro mycin in the spleens. Only six fish had detectable erythromycin in the live r and no muscle tissues collected had detectable erythromycin.