Reported are the results of a study in São Paulo, Brazil, to evaluate the bacterial contamination of the lacteal contents of feeding bottles prepared in urban households of low (LSE) and high (HSE) socioeconomic groups, involving 100 and 32 mothers of infants, respectively. Samples of the lacteal contents of the feeding bottles were cultured and the medians (25th and 75th percentiles) of the counts (bacteria per ml) were significantly higher in the LSE group: mesophilic bacteria, 555,000 (17,250-4,350,000) in the LSE group and 1615 (20-500,000) in the HSE group; coliforms, 2400 (19-150,000) in the LSE group and 7 (0-7800) in the HSE group. Escherichia coli was isolated from 26% (26/100) of the samples from the LSE group and from 6% (2/32) of those from the HSE group (P = 0.03). In the HSE group, higher coliform counts were associated with foodhandlers other than the mother, lower levels of maternal education, the use of pasteurized milk, and the addition of ingredients other than milk. In the LSE group, feeding bottles prepared using tap water and those prepared for infants aged over 6 months had higher coliform counts. In general, the feeding bottles prepared in the households studied were heavily contaminated, especially in the LSE group.