Equitoxic doses (10% clonogenic survival) of UV radiation (UVR) from the three waveband regions, i.e. UVAl (340–400 nm), UVB (290–320 nm) and UVC (200–290 nm), were shown to induce immediate or delayed apoptosis in L5178Y-R murine lymphoma cells. Membrane and DNA damage were shown to be the most probable initiators of UVAl-induced immediate or UVR-induced delayed apoptosis, respectively. These UV-induced apop-totic processes appeared to utilize two different “core” biochemical mechanisms; however, one core mechanism could be initiated at two distinct sites (e.g. membrane or DNA) and result in disparate kinetics. In an attempt to resolve this mechanistic issue, the dependence on mac-romolecular synthesis of each UV-induced apoptotic mechanism was investigated. In the absence of UVR, inhibition of either transcription (actinomycin D) or translation (cycloheximide) induced apoptosis in a concentration- and time-dependent manner. These results suggest that an apoptotic mechanism exists that does not require macromolecular synthesis postinsult (constitutive). The UVR data demonstrate that UVAl-induced immediate apoptosis utilizes this constitutive mechanism (preprogrammed), while UVR-induced delayed apoptosis utilizes the well-known inducible mechanism (programmed). Therefore, there are two different core biochemical mechanisms of apoptotic death available to each cell: preprogrammed (constitutive) and programmed (inducible) cell death.