Synergistic interaction between (a). IBMX or (b). rolipram and PGE2 on fibroblastic colony formation by whole bone marrow. Whole bone marrow was prepared and cultured in the CFU-f assay as described in the text in the presence of either (a). 10 μM IBMX or (b). 10 μM rolipram in combination with varying concentrations of PGE2. The medium was changed for fresh, PDE inhibitor & PGE2-free, medium after 5 d and thereafter twice weekly. The cultures were then stopped after 18 d, fixed with cold ethanol and stained for total colonies with methylene blue. They were then photographed and quantitated. Data presented are mean colony numbers per Petri dish ± S.D. * Denotes significant difference from control cultures, p < 0.05; # denotes significant difference from corresponding IBMX or rolipram-free cultures, p < 0.05.