On the basis of the findings of the present study, it is apparent that, at least in one city, infant mortality is no longer the extremely sensitive index of differences in socio-economic status that it was in the past. The erratic pattern that was observed when total infant mortality was related to socio-economic status was found to be a reflection of the fact that the majority of the deaths under one year of age today occur during the 1st few hours or days of life, and are due to causes that are associated with biological rather than socio-economic factors. On the other hand, for those few deaths that take place between the ages of 1 month and 1 year, where the major causes of death are farther removed from the physiological processes of gestation and birth, infant mortality continues to be inversely related to differences in socio-economic status.