In a comparison between male and female broiler chicks, the mortality rate of males was found to be significantly higher than that of females, starting from the second week of age until marketing at 7 or 8 weeks of age. The main causes of death during this period were various infectious diseases. This observation was explained by differences in the activity of humoral and cell-mediated immune responses between the sexes. In tests of antibody responses of young chicks to a variety of antigens (bacterial-E. coli, viral-Newcastle disease virus, and protein antigen-bovine serum albumin), females responded 24-72 h earlier than males and with higher peak antibody titers. In-vitro proliferation of T-lymphocytes to purified protein derivative and E. coli showed an earlier and greater response in females. The correlation between immune responsiveness and survival, as tested by challenging vaccinated chicks with pathogenic E. coli, showed a significantly higher mortality rate in vaccinated males, that was correlated with their lower antibody titer. We concluded, therefore, that sex-related differences in mortality rates of broiler chicks may result from a less efficient immune response in males.