The role of endogenous prostanoid synthesis in PDE4 inhibitor-mediated stimulation of colony formation. To test the hypothesis that some of the PDE4 inhibitors were also acting via the stimulation of endogenous prostanoids synthesis, CFU-f cultures were challenged with either EMD95833, EMD273316, EMD219906 or EMD249615 (1 μM) in the presence or absence of 3 μM indomethacin (a.). Medium changes, fixation and staining were as in Fig. 1. In addition, immediately before the first medium change, 1 ml aliquots of medium from were removed and PGE2 levels determined by ELISA (b.). Data presented are means ± S.D. * Denotes significant difference from control cultures, p < 0.05, # denotes significant difference from corresponding indomethacin-free cultures, p < 0.05.