A flow cytometric method has been developed for rapid determination of sperm concentration in semen from various mammalian species.* All cells containing DNA are stained with SYBR-14 or propidium iodide (PI) and sperm concentration is determined in relation to an internal standard of fluorescent microspheres ( beads). Satisfactory staining can be achieved within 2-3 min and the following flow cytometric analysis on the FACSCount AF System rapidly provides the user with a precise and accurate assessment of the sperm concentration. In this study, the FACSCount AF System and Sperm Counting Reagent ( BD Biosciences) was compared with microscopic counting using a Burker-Turk haemocytometer. In addition, sperm concentration was determined using the Corning 254 spectrophotometer which is used routinely by Danish artificial insemination stations for boars. The results show that the agreement between flow cytometry and microscopic counting is very high. The slope for the regression line was 1.12 (SE = 0.03) with an estimated intercept with the Y-axis of 22 x 10(6) sperm/ml (SE = 10 x 10(6) sperm/ml) and an estimated error of the model of 10 x 10(6) sperm/ml. For the spectrophotometer, the slope of the regression line was 1.09 (SE = 0.07) with an estimated intercept of 137 x 10(6) sperm/ml (SE = 25 x 10(6) sperm/ml). The average error made by the spectrophotometer was 55 x 10(6) sperm/ml. In addition, the results obtained using flow cytometry was highly repeatable ( CV = 2.7%) in comparison with the spectrophotometric method ( CV = 6.3%). These results indicate that the FACSCount AF System is a valuable tool for precise and accurate assessment of sperm concentration in boar semen and that use of this system may lead to production of more uniform insemination doses containing a specific number of sperm per dose.