Changes in mean red cell count (RCC), mean cell volume (MCV) and plasma alkaline phosphatase activity (ALP) were observed in a group of 33 normal male university students at a 7-week interval. Linear regression analysis shows a negative linear relationship between changes in RCC and MCV (r = -0.77, P less than 0.0005) while a positive correlation was found for changes both in ALP activity and in MCV (r = +0.37, P less than 0.025). Multiple regression analysis indicates that 70% of the variation in MCV could be accounted for by changes in RCC and plasma ALP activity (r = +0.84, P less than 0.0025).