A new pregnane derivative, 12β-hydroxy-5α-pregn-16-ene-3,20-dione [1], along with the known derivative 12β-hydroxy-pregna-4,16-diene-3,20-dione [2] have been isolated from a MeOH extract of the stem of Gelsemium sempervirens and found to be the principal cytotoxic entities. The ¹³C-nmr spectra of both compounds were assigned by comparison with other pregnane analogs thereby allowing confirmation of the stereochemistry at C-5 in compound 1. Heteronuclear 2D correlation and selective INEPT experiments indicated the need to revise a number of ¹³C-nmr assignments of pregn-4,16-dien-3,20-dione. Nine indole alkaloids, gelsemine, gelsevirine, 21-oxogelsemine, gelsedine, 14β-hydroxygelsedine, gelsenicine, humantenidine, humantenirine, and koumidine were found to be inactive in the KB and P-388 cytotoxicity test systems.