Human sperm motility was spectrophotometrically quantitated by a technique based upon the optical anisotropic properties of motile sperm. A sperm motility index [SMI] was determined and controlled experiments showed that the index was positively correlated to the percent of progressively motile cells (r = 0.92) and negatively correlated to the percent of heat killed cells in the sample (r = -0.92). In a single dilution of sperm cells with aliquots studied at selected time intervals for 90 min., the index correlated with a decline in the progressive motility (r = 0.95). The correlation between the index and the percent progressive motility in the fresh dilution was r = 0.86. When the index was determined on the same original population of spermatozoa, but experimentally prepared with different concentrations of cells, the index was independent of final cell numbers in a range of 2.5-14.3 .times. 106 cells/ml. Semen analyses were performed on each ejaculate and a correlation between the SMI and progressive motility (r = 0.85) was observed.