Repeated experiments examined the effect of castration on number of RBC, WBC, haemoglobin concentration and body weight in mice, Mus musculus. Castrations in general have been reported to lower the RBC, WBC and haemoglobin concentration but increase in body weight. The body weight of the castrated mice showed an increase tendency in comparison to the normal mice with a percentage of 28.51. In the castrated mice the number of RBC, WBC and haemoglobin concentration per 100ml of blood were found less than the normal intact mice. Thus, it appears that the male sex hormone (testosterone) exert influence on bodyweight, RBC, WBC, and haemoglobin concentration in a significant manner.