5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is an antivascular drug that induces tumor necrosis factor (TNF) in mice. Thalidomide inhibits TNF induction by DMXAA and also potentiates its antitumor activity. We investigated whether these effects were enantiomer specific, using the R- or S-enantiomers of two nonracemizable thalidomide analogues. Racemic 3-fluorothalidomide (3FThal) and racemic 3-methylthalidomide (3MeThal) were separated into enantiomers of greater than 98% optical purity using preparative chiral column chromatography. C57Bl/6 mice implanted with subcutaneous Colon 38 tumors were treated with DMXAA (25 mg/kg) alone or together with the pure R- or S-enantiomers by a single i.p. injection. TNF levels in the serum or tumor tissues 3 h after treatment were measured using ELISAs and tumor growth was also measured. 3FThal and 3MeThal, at their respective single maximum tolerated doses (MTD) of 15 and 50 mg/kg, were more toxic in mice than thalidomide (100 mg/kg). The R- and S-enantiomers of either 3FThal or 3MeThal, at their respective MTD, inhibited DMXAA-induced TNF activity in serum and tumor tissue, but no significant differences were observed between the enantiomers. Coadministration of racemic or enantiomers of 3FThal or 3MeThal at their respective MTD did not potentiate the antitumor responses above that obtained with DMXAA alone, and no enantioselectivity was apparent. We conclude that there is no advantage in using the nonracemizable thalidomide analogues to improve the antitumor activity of DMXAA.