Dual-parameter (DNA, RNA) flow cytometry (FCM) measurements were made on testicular and epididymal sperm cells isolated from mice exposed by oral gavage to 0, 250, 500, or 1000 mg/kg X 5 d of the fungicide methylbenzimidazol-2-yl carbamate (MBC), which is known to bind with tubulin subunits and inhibit polymerization and microtubule formation. Effects of exposure to MBC were measured at 7, 24, and 39 d posttreatment. MBC had no effect on body weights, but testis weights and sperm parameters were altered, with few exceptions, only at the highest exposure level. Testis weights were reduced by about 25% at 7 and 24 d after exposure; recovery was observed by 39 d after treatment. FCM measurements of testicular cells showed relative percentages of certain testicular populations (round, elongating, and elongated spermatids) were different from the control pattern 7 and 24 d after treatment. The mean percent of cauda epididymal sperm head morphology abnormalities and the susceptibility of the nuclear DNA to denaturation were both elevated at 7, 24, and 39 d after exposure to 1000 mg/kg. The level of denaturation was determined by FCM measurements of the metachromatic shift in acridine orange (AO) stained sperm nuclei from green (native DNA) to red (single-stranded DNA) fluorescence and quantitated by the expression alpha t[red/(red + green] fluorescence. These data demonstrate that spermatogenesis is sensitive to high-dose MBC exposure resulting in an altered ratio of testicular cell types present, abnormal sperm head morphology, and an altered sperm chromatin structure.