Comparison of the differentiation-inducing activity (NBT reduction) of retinoids. HL-60 cells (top panel), NB4 cells (middle panel), and fresh APL cells (lower panel) were cultured with 10^-7 mol/L of either ATRA, Retinoids A, B, C, D, E, or 10^-7 mol/L ATRA combined with 10^-7 mol/L of one of the other retinoids for 5 days. Differentiation was determined by NBT reduction. Results are expressed as the percentage of control dishes that contained no retinoids (100% activity) and represent the mean ± SD of three independent experiments performed in triplicate dishes.