Receptor-like tyrosine kinase IRR (the insulin receptor-related receptor) can be activated by extra- cellular alkaline media. IRR is found in organs that come in contact with liquids of extremal pH, and also in specific cells of the nervous systems where its function is not known. In this study, we analyzed the phenotype of IRR knockout mice in a series of behavioral tests. In control experi- ments, null-mutation littermate mice were analyzed. In the "Social interaction" test, the knockout animals showed a reduced number of social contacts. No statistically significant differences in im- mobility time were revealed in the "Forced swim" test, yet the number of animals that showed pro- longed immobility time, was higher in the group of knockout mice. In the "Resident-intruder" test, wild-type mice demonstrated their typical aggressive behavior whereas 7 out of 16 knockout animals stayed inert and, in contrast, attacked by the intruder. The obtained data suggest that the IRR gene inactivation results in disturbances of the aggressive-defensive behavior typical of the parental mouse strain.