It is generally accepted that serum albumin levels are the best criterion of protein nutritional status. In order to investigate this situation in the Black populations of South Africa, serum albumin concentrations were determined in several hundred normal individuals during each of three large-scale epidemiological surveys. The populations, who lived at very different socio-economic levels, were a tribal Xhosa group, a rural Tswana community and a sample of the detribalized urban population of Soweto. In each group males and females had similar serum albumin levels, which decreased with advancing age; however, there were significant overall differences between each of these populations (P less than 0,05). In distinction to the findings in other surveys, the rural group had the highest serum albumin level, while the tribal population had the lowest. It is apparent that the semi-westernized rural Tswana group enjoy an adequate standard of protein nutrition, the urban community is in a marginal situation, and the tribal Xhosa group is seriously undernourished.