The hypoosmotic swelling test(HOST) and acrosin activity assay were tried on ejaculates from fertile and infertile men. The results of HOST were significantly higher in fertile men. They correlated with sperm motility in fertile and infertile men. We can infer from these results that HOST may be a reliable method of detecting sperm fertilizing capacity. However, acrosin activity assay, using the spectrophotometric method, seems unreliable to detect this capacity.