A topical, aqueous gel formulation containing the mebutate salt form of ingenol, a selective small-molecule activator of protein kinase C (PKC) that is isolated from the sap of Euphorbia species, with potential antineoplastic activity. Upon topical application of the ingenol mebutate gel, ingenol activates various PKC isoforms, which induces apoptosis in certain tumor cells, including myeloid leukemia cells, melanoma cells, and basal cell carcinoma cells. The PKC family consists of signaling isoenzymes that regulate many cell processes, including proliferation, differentiation, and apoptosis.