Brucellosis is an important and widely distributed zoonosis in Mexican cattle which also affects an unknown proportion of the human population. This report presents the brucellosis antibody levels registered in the National Seroepidemiology Survey (NAS) in sera obtained from 66,982 healthy persons from one to 98 years of age and determined by the test of plaque microagglutination. Seroprevalences by states ranged from 0.24 per cent in Morelos to 13.5 per cent in the state of Mexico. The national mean was estimated to be 3.42 per cent. The analysis showed no statistical differences for brucellosis antibody levels by urban and rural residence and by density of family sleeping areas (three or more persons vs. one or two persons per bedroom). Adults between 20 and 39 years of age had greater seropositivity and children from one to nine years had the least. Women were most affected and had 48 per cent more seropositivity than men. According to the information obtained in the study, brucellosis in Mexico has the following characteristics: it is related to gender but not to occupation; affects persons in all age groups, social strata and is independent of size of the community of residence. Historically, brucellosis has been an endemic disease in Mexico. Recently an increasing incidence has been reported, and this is possibly due to a better national notification system.