1. Blood Counts and Hæmoglobin determinations on 60 young male adults living on the Witwatersrand (altitude 5750 feet) gave the following results:—Red Blood Corpuscles.Range . . . . 5,120,000 to 6,820,000.Mean probably lies between 5,800,000 and 6,160,000.Hœmoglobin.Range . . . . 88 to 118.Mean probably lies between 103 and 108.Colour Index.Range . . . . 0·81 to 1·12.Mean is probably not significantly different from 1.2. Tables are given showing the probable error of a single blood count and the frequency distribution of the red blood cells, hæmoglobin and colour index in the subjects tested.