The discovery of the reversible antifertility action of an extract from Tripterygium wilfordii both in male rats and in men in 1986 stimulated worldwide interest. International and national collaborations aimed at the bioassay-directed sub-fractionation of materials extracted from the plant was then organized and to date, a series of six male antifertility diterpene epoxides have been isolated. Their chemical structures have been identified and found to be triptolide, tripdiolide, triptolidenol, tripchlorolide, 16-hydroxytriptolide and T7/19 (structure not yet published). At the ED95 dosage levels, they act mainly on metamorphosing spermatids and testicular and epdidymal spermatozoa with exfoliation and inhibition of basic nuclear protein turnover of late spermatids, delayed spermiation and sperm head-tail separation and microtubule, microfilament and membrane damages. A preliminary toxic evaluation indicated that these compounds were immunosuppressive at dose levels 5-12 times their antifertility doses. Immuno-suppression is an important weakness for an antifertility agent, but if the immuno-suppressive dose of a drug is much higher than its antifertility dose, it could yet be regarded as a safe contraceptive. Therefore, in the safety evaluation of compounds isolated from Tripterygium wilfordii, it warrants our attention to probe deeply into their precise dose/immuno-effect relationship.