Introduction Since the discovery of and the subsequent use of griseofulvin in the systemic treatment of dermatophyte infection, many observations have been made concerning this antibiotic's effect on both the host and the infectious organism. It seems clear that orally administered griseofulvin inhibits the growth of dematophytes in vivo without inducing significant adverse effects an either man or experimental animals.2-6 When this antifungal antibiotic is injected intravenously in rats at approximately ten times (100-200 mg/kg.) the oral therapeutic dosage, arrest of mitosis is observed in metaphase.7 In this respect griseofulvin resembles colchicine. Similar effects by griseofulvin have been reported in plant cells.7 This paper presents data on the blood and serum levels of mice following a single oral dose of griseofulvin, chromatographic and spectrophotometric assay techniques for this antibiotic, and the partial reversal of griseofulvin activity by purines, pyrimidines, and their nucleotides. Methods Assays Utilizing Tritium-Labeled Griseofulvin.—