Pulse-exposure of HL-60 cells to either 1,25D₃ (□) or Ro-25-9716 (⌾). HL-60 cells were exposed for various durations to the vitamin D₃ compounds (10⁻⁹ mol/L). The cells were then throughly washed three times, counted, plated (1,000 cells/well) into soft agar, and colonies were counted 14 days after plating. Each point represents the mean of three experiments with triplicate dishes per point.