This study was undertaken to investigate and compare in vitro and in vivo immune parameters between female and male rats. We analysed the T-cell proliferative responses to syngeneic and allogeneic cellular antigens (syngeneic and allogeneic mixed lymphocyte reaction), as well as the IgG levels in the sera of our study groups. It has also been studied the influence of gonadectomy and the effect of testosterone administration pre- and postnatally on the above parameters. Our findings showed that hormonal manipulations, can alter the differences observed in immune response between female and male rats. In addition, it is demonstrated that pre- and postnatal sexual steroid manipulations could provoke long lasting immunological effects.