Purpose: Bryostatin 1, a unique protein kinase C (PKC) activator, is already in the clinical trials. An understanding of complex regulation of PKC by bryostatin 1 is essential for effective use of bryostatin 1 in the clinic. We have previously shown that the ability of bryostatin 1 to enhance cisplatin sensitivity correlated with its ability to down-regulate PKCδ in HeLa cells. We have investigated how bryostatin 1 influences PKCδ regulation in cisplatin-resistant HeLa (HeLa/CP) cells, and if bryostatin 1 could be used to reverse cisplatin resistance. Experimental Design: Phorbol 12,13-dibutyrate (PDBu), bryostatin 1, and small interfering RNA were used to manipulate PKC level/activation status. Cell death was monitored by 3-(4,5-dimethylthiazol-2- yl)-2,5-diphenyltetrazolium bromide (MTT) assay, Annexin V dye-binding assay, and analysis of hypodiploid peak in a flow cytometer. Results: Bryostatin 1 elicited a biphasic concentration response on PKCδ down-regulation and cisplatin-induced cell death in HeLa/CP cells; the maximum effect was achieved with 1 nmol/L bryostatin 1. Down-regulation of PKCα increased with increasing concentrations of bryostatin 1. PDBu induced down-regulation of PKCα in HeLa and HeLa/CP cells but it had little effect on PKCδ down-regulation in HeLa/CP cells. However, both PDBu and bryostatin 1 enhanced the sensitivity of HeLa/CP cells to cisplatin. Knockdown of PKCδ by small interfering RNA inhibited cisplatin-induced apoptosis but knockdown of PKCα enhanced cisplatin-induced cell death. Conclusions: Those results suggest that although PKCδ acts as a proapoptotic protein, full-length PKCδ may inhibit cisplatin-induced cell death. Thus, persistent activation/down-regulation of PKCδ by bryostatin 1 was associated with cisplatin sensitization. Furthermore, PKCα acts as an antiapoptotic protein and down-regulation of PKCα by PDBu was associated with cellular sensitization to cisplatin.