A high-pressure liquid chromatographic assay for carboprost tromethamine as the bulk drug and in a sterile solution formulation is described. The procedure involves derivatization of the prostaglandin to form the UV-absorbing naphthacyl ester, which then is chromatographed on a silica gel column using methylene chloride-1,3-butanediol-water (496:4:0.25) as the mobile phase. This procedure is compared with a nonderivatization procedure with refractive index detection. Both procedures separate the 15R-epimer of carboprost from carboprost, but only the derivatization procedure separates the 5-trans-isomer of carboprost. Possible reasons for the better separation using the derivatization procedure are discussed. Both procedures gave a coefficient of variation of approximately 1% for carboprost. The derivatization procedure gave a coefficient of variation of approximately 7% for the 15R-epimer and 5-trans-isomer when present at 2% of the carboprost level.