High density lipoprotein (HDL) cholesterol is a sensitive index for coronary disease in affluent societies. We have measured plasma apoprotein A-I levels (the major HDL protein) in randomly selected groups of urban and rural Fijian Melanesians and Indians. Despite higher prevalence rates of coronary disease and diabetes mellitus in Indians, Indian men and women had significantly higher A-I levels than Melanesian men and women. Multivariate analysis was carried out separately in all men and women of both races and also in younger men and women (less than 45 years old) to determine the predictive values of seven variables that might influence A-I levels. These variables accounted for about 16% of the A-I variation and of this, more than one-half was due to ethnic origin. The remainder was largely due to three environmental factors: urbanization, alcohol consumption, and physical activity. Men and women aged 20 to 44 years had significantly higher A-I levels in the town than in villages; alcohol drinkers had significantly higher A-I levels than nondrinkers; and in women physical inactivity resulted in significantly lower A-I levels. Age, smoking cigarettes, and body mass index did not contribute to the differences in A-I levels between the two races, despite less smoking and overweight among Indians. This study of a biracial population, that shares a similar environment but differs in cultural habits, has demonstrated the operation of genetic and environmental factors that explain a minor proportion of apoprotein A-I variability.