A relationship appears to exist between the morphological appearance of the seminal coagulum and the rate of its liquefaction. The guinea-pig and rhesus monkey coagula liquefy only poorly, and consist of thick fibres which form a solid structure. A normal human coagulum possesses an extensively organized network of long thin fibrous strands. A `slow liquefying' human ejaculate shows similar patterns although it possesses a multitude of thick fibres. As the coagulum liquefies, the fibres become disorganized and turn into spherical material. Spaces within the fibrous network of the human coagulum are so small that escape of spermatozoa does not seem possible without liquefaction. This may explain the subfertility of poorly liquefying or non-liquefying human semen.