The motility of ejaculated boar spermatozoa was assayed by measuring the number of organisms which have entered a capillary tube in an experimental setup with the spermatozoal suspension as well as the capillary content containing the same chemically defined motility medium. This medium, used both for washing and suspending spermatozoa, contained only an isotonic tris HCl (isotris) buffer at pH 7.0. The recommended assay conditions are the following: (1) wash the spermatozoa three times with isotris buffer, (2) suspend the spermatozoa in a concentration of 10(6) spermatozoa/ml, and (3) assay at 25 degrees C with an incubation period of 60 min.