There is not information, in our medium, about the incidence of Streptococcus beta hemolytic Group B, as causative agent in neo natal infections. Between march 1981 and January 1982, 200 mothers, admitted in the Hospital General de Medellin, and their infants, were studied by us, in order to obtain information about the prevalence of Streptococcus beta hemolytic group B in neonatal sepsis. Samples were taken from vagina, rectum and throat, in the mothers; and ears, rectum, umbilical cord, pharynx, in infants. Technics and culture mediums are described. Three mothers were positive (1.5%): Two for vaginal and one for vaginal and throat samples. Two newborns were positive: one of them for all samples and he is son of mother with vaginal and throat infection. Other positive infant, and a negative mother. The incidence of Streptococcus Group B, is low and in accordance with absence of clinical disease in our service. Contrary to informed by other authors, gestational age or precocious rupture of membranes, were no related with infection by Streptococcus Group B, in our medium at present. Socioeconomically status of affected mothers is commented.