A simplified quantitative cervical mucus penetration test was developed and applied to the semen of 21 prospective donors in order to evaluate its performance. Analysis of the data using stepwise multiple regression revealed that the concentration of spermatozoa in mucus was related primarily to the concentration of progressive spermatozoa in the semen (63% of variance explained), and secondarily to the average path velocity of spermatozoa in semen (70% of variance explained by a model incorporating both variables). However, consistent with previous studies, the derived indices of penetration efficiency including the percentage successful collisions (PSC), percentage successful entries (PSE) and the motile density ratio (MDR) were not significantly correlated with any of the semen variables examined, although multiple regression did derive models which explained 27-31% of the variance in these three dependent variables. The strong correlation and correspondence between the PSC and the MDR suggests that the latter may provide a simplified index of penetration efficiency for routine clinical use and further research in this area.