Abstract: The invention relates to improvements in determining the fertilizing capacity of a human semen sample using a reagent solution which contains 0.1-5% by weight K.sub.2 PO.sub.4 H/KPO.sub.4 H.sub.2 to adjust the pH of the solution to 6.5-8.0. After treatment with the reagent solution, the result is considered positive when the diameter of the halo is equal to or larger than 7.5.mu. in 10-25% of the spermatozoa read, as long as the membrane of the head of the spermatozoon is intact. Kits suitable for performance of the claimed method are also described.