Carefully timed and quantitated Kremer-type tests were performed for assessment of sperm-mucus interaction in 100 couples who underwent an infertility workup. A novel scoring system for these in vitro tests was elaborated and validated. The success of penetration of seminal spermatozoa into the mucus, and their migration within the mucus column, was related to both the sperm concentration and their motility, especially their progressivity. A calculated index of sperm motility quality gave a 70% accuracy rate in predicting the test result (normal versus abnormal), but this success rate was not significantly improved by incorporation of the sperm concentration into the index. No clear biologic relationship was found between this method for assessing Kremer-type tests (which correlated closely with accepted clinical reporting practices) and Katz's "percentage of successful collisions." This apparent discrepancy is considered to reflect basic differences in the approaches of the two scoring systems.