Various factors contribute to the formation of malodor in intravaginal collagen sponges (CS) worn for several days by sexually active persons. To study this problem we developed an in vitro model: CS was infiltrated with semen and incubated at various pH's or in the presence of various drugs, at 37° and 95% O2-5% CO2 in humid atmosphere. The odor was measured by subjective olfactometry as well as quantitated and characterized by gas liquid chromatography. Based on previous findings concerning the disappearance of polyamines from the semen in sponges with malodor and present detection of maximal malodor formation at pH 7.4 and absence of malodor with heat-inactivated semen we propose that malodor forms through the enzymatic oxidation of polyamines or diamines possibly by diamine oxidase (DAO) of the semen. Direct assay of DAO showed that all factors inhibiting or stimulating the malodor affected the activity of DAO in the same direction. Thus, while copper ions were stimulatory to odor and DAO activity, zinc ions, acid environment (3.4 pH), exogenous spermine administration, isoproniazid and pargyline, both known inhibitors of MAO's and DAO's, reduced malodor and DAO activity. We found that more intensive malodor developed in sponges made of polyurethane, cellulose or spongin (sea sponge) than of collagen. Whole semen caused greater malodor formation than prostatic fluid and seminal plasma. Both washed spermatozoa and vaginal secretion did not induce any subjectively detectable offensive odor in sponges. Addition of a mixture of nonpathogenic microorganisms to the semen did not enhance the formation of subjectively detectable malodor. GLC showed characteristic differences between volatile compounds formed by ejaculate and bacteria. While most of the malodor formed by the semen-CS system was enzymatically related, some evidence indicates a portion of malodor to be nonenzymatic process. It is concluded that the most feasible method preventing the semen related vaginal malodor is to keep the CS acidic (pH 5) and to impregnate sponges with low concentrations of zinc sulfate (40 mg/sponge), unless the sponge is removed 24 h after the intercourse, washed and reinserted.