Virginiamycin (Stafac 20) was mixed with feed at three levels recommended for chickens--5.5, 11, and 22 mg/kg (respectively 5, 10 and 20 g/ton)--and fed to broiler chicks. When fed from 1 day through 2 weeks of age, the drug appeared to retard infection of the lower small intestine by long, segmented, filamentous organisms (LSFOs), and at 2 weeks of age serum carotenoids in treated chicks were significantly higher than levels found in unmedicated chicks. However, as chicks were grown out to 4 and 6 weeks of age, the drug did not completely prevent eventual LSFO infection, even at the highest dose, and mean serum carotenoid levels in treated chicks were not significantly different from levels in control chicks. However, chicks fed at the level of 22 mg/kg had fewer LSFOs. Withdrawal of virginiamycin from treated chicks at 4 weeks of age allowed LSFO infection to occur but did not significantly affect serum carotenoid levels. When the drug was fed for 2 weeks at 22 mg/kg to 2-week-old chicks already infected with LSFOs, the bacteria could no longer be detected, suggesting that virginiamycin may aid the natural decline in LSFO population. Coincident with this treatment, serum carotenoids were higher, but not significantly so. Virginiamycin did not significantly increase the mean body weights of chicks in any of these experiments.