The quality of ejaculated human sperm was examined for 24 h at room temperature by Percoll continuous density gradient centrifugation. One hour after ejaculation, apparent densities of good motile sperm with normal morphology were above 1.10 g/ml, whereas those of immotile and/or immature sperm recovered from upper layers were 1.06-1.09 g/ml. As time passed, sperm became immotile and the number of sperm with higher densities was diminished, their density shifting to 1.06-1.09 g/ml. Morphological observations by transmission electron microscope showed that after 24 h the sperm with higher densities were morphologically normal, except that the plasma membrane and the outer acrosomal membrane were lost. On the other hand, the sperm with lower densities were abnormal, such as immature, without the outer and the inner acrosomal membrane, with the nuclear decondensation, and with membraneous structure in the nucleus. These results suggest that after ejaculation the apparent density of human sperm became lower, concomitant with the loss of motility.