Protein deficiency in the diets of underprivileged pregnant women in developing countries is widespread. Carbon monoxide (CO) pollution is serious public health problem in developed as well as in developing countries. Two-way factorial experiments were conducted to study mice pup mortality in different levels of maternal protein and various concentrations of CO. Pregnant CD-1 mice were maintained on 27, 16 (control), 8 or 4% protein diets throughout gestation. The dams were exposed to 0 (control), 65 or 125 ppm CO in air, in environmental chambers for 6 hours/day (6h/d) during the first week, or first the two weeks of pregnancy. The animals were allowed to deliver, and data on pup mortality was recorded. The birth weight of pups in one week and two weeks CO exposures was inversely related to CO concentrations and directly related to dietary protein levels. The pup mortality on birth day and at one week age in one week and two weeks CO exposures was directly related to CO concentrations and inversely related to dietary protein levels. The data suggest that protein deficiency and CO exposure enhance pup mortality. The protein and CO also interact to increase pup mortality in 16% and 27% protein groups. The data also suggest that unvented combustion for heating and cooking, ambient pollution, and biomass smoke may have a major impact on the health of children worldwide; and CO along with protein deficiency during gestation may be contributing factors to high infant mortality in poor countries and some sections of  the United States.