PIP: The characteristics and extent of and trends and differentials in infant mortality in Mexico are examined. Bivariate and multivariate analyses were based on data from the Encuesta Nacional de Fecundidad y Salud (ENFES) in 1987. The birth period was 1976-85, and the infant mortality reference period is June 1980. Analysis included breakdowns by social sector (agricultural and nonagricultural: middle-class nonmanual workers (25.5%), working-class manual workers (46.3%), salaried workers (33.5%), and nonsalaried workers (12.8%). Infant mortality is measured as the probability of death between birth and 2 years (2q0). The results show a decline in infant mortality, but the rate of decline is unequal among social strata. There was a 11.4% decline between 1976-85 in 2q0. The patterns were as follows: infant mortality is higher at 34 and 20 years; higher order pregnancies had higher infant mortality; longer average time between pregnancies yielded lower infant mortality; 2q0 is greater in towns of 20,000 population and lower in metropolitan areas; 2q0 is higher among women with little or no education; 2q0 among women with a negative score on the Index of Housing Services (INSEVI) is twice that of women in the middle and higher categories; and 2q0 is higher among women in the agricultural sector. There was a 17% rise in 2q0 (from 73.8 to 86.2 deaths/1000) between 1972-80 in the agricultural sector, and the nonagricultural sector showed uneven declines. Among women in the middle-class nonmanual sector there was a decline of 18% (27.9 deaths/1000; among working-class nonsalaried workers, 12%; and among working-class salaried workers, 5% (48.6 deaths/1000). In the analysis of differentials, 1) age of mother at time of birth, 2) mother's education, and 3) the INSEVI index of services available in the dwelling confirmed the presence of differentials in infant mortality. For total population, age of the mother at time of birth shows a typical U-shaped pattern, but the social sector analysis shows that the age mortality relationship is inverse when good services are available in the dwelling, with the exception of working-class nonsalaried workers. In the agricultural sector, 2q0 of mothers aged 15-19 is 10 times higher than women aged 35-39 with similar services in the homes. The multivariate model reveals that mother's education is not significant in all models. The relationship between size of place of residence is inversely related to mortality only among women in the middle-class nonmanual workers. In all nonagricultural sectors, availability of adequate services has a strong effect on reducing 2q0. Age of mother at time of birth and birth order are the only variables significant in all 4 models.