Ethidium bromide (10 mug/ml) and bromodeoxyuridine (25 mug/ml) were used to sensitize selective cell organelles to visible wavelengths of an argon ion laser (488 and 514 nanometers). Ethidium bromide was shown to be selective in sensitizing nucleoli, chromosomes, and the centriolar region of PTK2 cells to the laser microbeam. Similarly, BrDU sensitized chromosomes to the microbeam irradiation. The lesions produced on the chromosomes when either agent was used appeared as a phase paling of the irradiated segment. Nucleolar lesions also appeared as a phase paling, and the centriolar region alteration appeared either as a phase paling or a phase darkening.