We have examined the effect of the macrocyclic lactone protein kinase C (PK-C) activator bryostatin 1 on the in vitro radioprotective capacity of recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) toward normal committed myeloid progenitor cells (day-14 granulocyte-macrophage colony-forming units [CFU-GM]). Preincubation of T-cell- and adherent cell-depleted bone marrow mononuclear cells with 12.5 nM bryostatin 1 and either 1.25 or 50 ng/ml rGM-CSF for 24 h resulted in an 18%-30% survival at 4-5 Gy, whereas cells exposed to rGM-CSF alone gave rise to no detectable colonies at radiation doses greater than 2.5 Gy. Coadministration of bryostatin 1 also led to a threefold increase in Do values for both rGM-CSF concentrations. A similar enhancement of radioprotective effects was observed with the tumor-promoting phorbol ester phorbol dibutyrate. Exposure of cells to both bryostatin 1 and rGM-CSF immediately following irradiation also resulted in enhanced progenitor cell survival when compared to rGM-CSF alone, but radioprotective effects were less than those observed when cells were preincubated with these factors. Cells preconditioned with bryostatin 1 and rGM-CSF prior to exposure to 2 or 4 Gy gave rise to significantly more colonies when radiation was administered as a 4-h divided dose, suggesting that bryostatin 1 may act by potentiating rGM-CSF-induced repair of sublethal radiation damage. Finally, pre-exposure of enriched progenitor cells (CD34+) to bryostatin 1 and rGM-CSF resulted in radioprotective effects that were less than those observed for partially purified populations with respect to the total population of surviving myeloid colonies. However, CD34+ cells preincubated with bryostatin 1 and rGM-CSF prior to irradiation exhibited a significant increase in both the percentage and absolute number of neutrophilic and macrophage colonies, and a reduction in eosinophilic colonies, compared to cells exposed to rGM-CSF alone. These studies suggest that bryostatin 1 (and possibly other PK-C activators) potentiates the in vitro radioprotective effects of rGM-CSF and may also regulate the lineage specificity of this response.