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The absolute photoelectron quantum yield spectra for benzo[ajpyrene and dimethylbenzanthracene are presented in the wavelength range 180-230 nm. These polycyclic aromatic carcinogens have photoelectron quantum yields of approximately 2 X 10-3 electrons per incident photon at 180 nm. The quantum yields fall off quickly and monotonically at wavelengths longer than 210 nm (5.9 eV). Threshold values for benzo[a]pyrene and dimethylbenzanthracene are 5.25 ± 0.06 eV and 5.27 ± 0.04 eV, respectively. The photoelectron quantum yields of benzo[a]pyrene and dimethylben-zanthracene are several orders of magnitude greater than typical components of biological membranes (amino acids, phospholipids, and polysaccharides). Preliminary micrographs of benzo[a]pyrene and dimethylbenzanthracene sublimed onto poly(L-lysine) and onto dimyristoyl phosphatidylcholine demonstrate the high contrast of small crystallites of carcinogens against a background of membrane components. These results and calculations involving relative contrast factors suggest that the distribution of these carcinogens in biological membranes can be determined by using photoelectron microscopy.