A variety of normal and transformed avian and mammalian cells and tissues contain high affinity specific receptors on their membranes which interact with biologically active phorbol and

ingenol esters in a reversible and saturable manner. The binding of labelled phorbol-12,13-dibutyrate (PDBu) to live or glutaraldehyde-fixed cells is dose-, time- and temperature-dependent.

Those phorbol or ingenol esters which stimulate cell growth in culture and have tumour-promoting activity in vivo inhibit the binding of labelled PDBu, while the biologically inactive

derivatives fail to do so. Other non-diterpene tumour-promoting agents, epidermal growth factor (EGF), retinoids and prostaglandins do not compete for the binding of labelled PDBu to its