The methanol extract (ME) and the n-butanol fractions of methanolic extract of Alstonia macrophylla Wall ex A. DC leaves were investigated on the forward motility (FM) of mammalian (goat and human) spermatozoa. The ME at 600 microg mL-1 as well as fraction B at 100 microg mL-1 concentrations showed marked inhibition of sperm FM in both goat and human species when tested by microscopic and spectrophotometric methods. Approximately 60-80% of the goat spermatozoa lost their FM when treated with 600 microg mL-1 of ME and 100 microg mL-1 of fraction B. At 100 microg mL-1 concentration, fraction B showed 90% loss of FM in human spermatozoa, while fraction B at 400 microg mL-1 concentration showed complete inhibition of sperm FM at 0 min. The inhibitory activity of fraction B increases with increasing concentration in a dose-dependent manner. Phytochemical study of the extract revealed that the leaf contains tannins, flavonoids, sterols, triterpenes, alkaloids and reducing sugars. Further fractionation and purification of the bioactive n-butanol part of ME showed the presence of ursolic acid (fraction B), beta-sitosterol (fraction A), beta-sitosterol glucoside and a mixture of minor compounds (fraction C, detected on thin-layer chromatography). The results reveal that fraction B (ursolic acid), a pentacyclic triterpene, has the potential of sperm motility inhibition and can serve as a topical vaginal contraceptive.