In México, since may 1986, the mandatory detection of antibodies against HIV in blood and derivates for the use by humans was legislated, one year later the prohibition of blood trade took effect. The impact of the Sanitary Normativity in the seroprevalence of anti HIV antibodies between blood donors one year before and one year after it took result was compared. A sample was taken from 4743 serums of donors of 6 blood banks in Guadalajara between june 1986 and june 1988; 4020 in the first year and 723 the next. In all the samples the presence of anti HIV antibodies was determined by the ELISA assay and the positive ones were confirmed by indirect immunofluorescence and by Western blot. During the first year, the seroprevalence fluctuated between 2.02 and 30.15 percent depending on different factors, differing from the second year where the seroprevalence was 0.14 percent. It is concluded that the mandatory detection of antibodies against HIV and the prohibition of blood trade, have been definitive factors to decrease the incidence of HIV in blood donors.