Young male CBA/LacStoRap mice for 2 h were exposed to pheromones that are transferred by major urinary proteins of sexually mature males of the same strain. The treatment was conducted using either a complete set of the major urinary proteins typical of the CBA mice or incomplete set of protein fractions detected in some animals. The effect of pheromones was estimated 24 h after treatment by cytogenetic analysis for disturbances in dividing bone marrow cells at anaphase-telophase and in germ cells at metaphase I. The frequency of both mitotic and meiotic disturbances was significantly increased after exposure to pheromones associated with the complete spectrum of the major urinary proteins. Conversely, no cytogenetic effect was observed in the absence of particular protein fractions. Possible consequences of the pheromonal effect on the genomes of somatic and germ cells are discussed as well as the relationships between pheromones and the major urinary proteins.