The protective effect of breast milk from 150 women on infection produced by protozoa Cryptosporidium parvum of children under 2 years old living in a shantytown nearby Lima, Perú was studied. Breast milk had specific secretory Inmunoglobulin A (sIgA) antibodies to C. parvum antigen determined by Inmunoelectrotransfer blot (EITB) assay. Each mother secreted milk with different composition of specific secretory IgA antibodies against 15 proteins of C. parvum antigen with molecular weight 15-158 k-Da, remained constant during the 24 month lactation period. The risk of C. parvum infection was reduced in children breast milk with specific secretory IgA against proteins of high molecular weight C. parvum antigen (158, 123 y 97 k-Da), protecting the breast fed baby from infection