7,12-Dimethylbenz(a)anthracene (DMBA) and 7-methoxymethyl-12-methylbenz (a)anthracene (MeO-DMBA) are converted to a number of products during short exposures in aqueous suspension to laboratory illumination. The mixture of products binds to glyceraldehyde-3-phosphate dehydrogenase (GPDH) while inhibiting its activity but there is no apparent relationship between the binding and inhibition of enzyme activity. There is little, or no, binding or enzyme inhibition when the compounds are protected from light. 7-Bromomethyl-12-methylbenz(a)anthracene (Br-DMBA) binds to GPDH whether photoactivated or not but enzyme inhibition depends upon light exposure. The binding of light-exposed DMBA by surviving rat mammary tissue was five-times greater than with the unchanged hydrocarbon. Binding of MeO-DMBA products also occurred after light exposure but not in the dark. © 1976.