The rate of racemization of N(alpha)-phthalimidoglutarimide (thalidomide) was determined as its half life to be 566 min at pH 7.4/37 degrees C. This fast racemization of thalidomide resulted in no apparent difference between (S)- and (R)-forms of the compound on enhancing activity of phorbol ester-induced tumor necrosis factor (TNF)-alpha production by human leukemia HL-60 cells. Optically pure forms of structurally related analog of thalidomide, (S)- and (R)-alpha-methyl-N(alpha)-phthalimidoglutarimides (methylthalidomides), which do not racemize under the physiological condition, were prepared. Only (S)-form of methylthalidomide, but not its (R)-form, elicited TNF-alpha production-enhancing effect, suggesting that the (S)-isomer of thalidomide would be the active form in terms of thalidomidal biological response modifying effects.