To investigate the effects of caloric restriction combined with physical training on the immunity in athletes, some cellular and humoral immune functions were examined in 9 young female athletes before and after a caloric restriction program. The control group consisted of 7 age-matched female non-athletes who were not on diet. The athletes continued their exercise trainings as usual and ate low-caloric diets (1, 300 kcal/day : including 60 g/day of protein) for 2 weeks. They reduced their body weights by 3.1% and their body fats by 18.2% at the end of the program. As the marker of nutritional condition, serum levels of pre-albumin and retinol binding protein were measured. Both of them at the end of 2 weeks were slightly decreased, but did not reach statistical significance, these athletes, however, showed slight hemoconcentrations after the caloric restriction program combined with physical exercise. Therefore, the total amounts of those two substances were supposed to be significantly decreased. The phagocytic activities of blood monocytes against sheep erythrocytes opsonized with IgG and saccharomyces cerevisiae yeast in the athletes were already higher than those in controls, even before the caloric restriction. After reducing their body weights, those phagocytic activities were significantly decreased (p<0.01, respectively) to the same levels of those activities in controls. The blastgenic response of T lymphocytes isolated from the athletes to the phytohaemagglutinin was also higher than the response in controls before the caloric restriction, and continued the same levels during the caloric restriction period. The complement activity and the serum concentrations of IgG, IgM, and IgA in athletes were not different from those in controls before and after the caloric restriction. The concentration of fibronectin in sera of the athletes were significantly decreased after the caloric restriction program (p<0.05) ; however, these concentrations were not significantly different from those in sera of controls. We concluded that even a moderate caloric restriction program might reduce some immune functions in the athletes continuing their daily physical trainings. The deterioration in immunity, however, might not affect their health, because those functions in the athletes of this study had been originally enhanced, and maintained the normal range after a moderate weight reduction.