WIN55212-2 did not enhance PMA-induced apoptosis. Differentiation of U937 cells was induced by 25 nM PMA with or without co-incubation with WIN55212-2. The experimental groups were: 1.) Vehicle control = treatment with 0.1% DMSO only, 2.) Differentiation control = treatment with 25 nM PMA, 3.) Experimental groups were treated additionally with 3 μM or 5 μM WIN55212-2, respectively. Differentiated (a) and non-differentiated cells (b) were separated due to its adherence after 72 h incubation and apoptotic cells were detected by annexin V staining and FACS analysis (figure 3a) or immunocytochemical analysis after counterstaining with DAPI (figure 2b). Annexin V-positive cells in the non-differentiated cell fraction are quantified in figure 3a and displayed in figure 3b. Immunocytochemical analysis of adherent cells are shown in figure 3c. Data are shown as mean +/- SEM, n = 12; *p < 0.05 in comparison with the total cell number. Scale bar = 10 μm.