1. The effects of human semen on rat descending colon fluid absorption, permeability to 3H-labelled polyethylene glycol 4000 and the histological appearance of the mucosa were examined. Also, the semen was fractioned by centrifugation into plasma and sperm fractions and the effects of these fractions on rat colonic function were examined. The effects of trypsin and bacterial collagenase, mimetics of acrosin and seminal collagenase activity, were examined in order to investigate which component of human semen alters colonic permeability. 2. Contact between human semen and rat descending colonic mucosa for 3 h decreased fluid absorption from 52.0 +/- 2.9 microliters h-1 cm-2 (control) to 10.7 +/- 3.4 microliters h-1 cm-2 (P less than or equal to 0.001), increased the permeability to polyethylene glycol 4000 from 0.099 +/- 0.006 cm/h (control) to 0.31 +/- 0.04 cm/h (P less than or equal to 0.001) and caused cytolysis of the surface mucosa. 3. Spermatozoa inside the colonic lumen were destroyed within 1 h with release of acrosomal contents; this raised the activity of the acrosomal proteolytic enzyme acrosin by 40-fold (P less than or equal to 0.005) and of seminal plasma metalloproteinase (collagenase) by about twofold (mean activity 1623 +/- 240 units/ml of luminal fluid). 4. The changes in colonic permeability induced by seminal plasma were similar to those induced by similar activities of clostridial collagenase. 5. We conclude that seminal collagenase is present in sufficient amounts to cause acute damage to the colonic mucosa, and that this could be a factor in facilitating viral transmission across the colonic wall.