The head shapes of mammalian sperm have been measured by slit-scan flow cytometry (SSFCM). In this approach, the distribution of fluorescence along acriflavine stained mammalian sperm is recorded and used as a measure of head shape. Fluorescence profiles were measured for sperm from mice, rabbits, hamsters, and bulls, and for sperm from mice exposed to testicular x-irradiation from 0 to 900 rads. The profiles for sperm from nonirradiated animals were characteristic of each species and were reproducible from sperm to sperm. Some of the fluorescence profiles for sperm from the irradiated mice differed significantly from the profiles usually measured for sperm from exposed mice. An algorithm was developed to determine the frequency of these sperm. The estimated frequencies of atypical profiles correlated well (r = 0.99) with the frequencies of abnormally shaped sperm determined by microscopic scoring. The maximum SSFCM sensitivity (minimum detectable dose = 199 rad) was not as high as that for the visual assay (minimum detectable dose = 116 rad). However, only 100 profiles were measured by SSFCM at each dose while at least 500 sperm were scored visually at each dose. The sensitivity of the SSFCM assay should be increased substantially by measuring more profiles. The objective nature of SSFCM couple with the high correlation with results from the visually based assay of morphology suggests the use of SSFCM to measure frequencies of misshapen sperm when testing for mutagens or monitoring for effects of environmental contaminants.