The titers of serum antibodies to natural infection with enteric and respiratory pathogens, to a food antigen and to tetanus and diphtheria toxoid were evaluated by enzyme-linked immunosorbent assay in 1,554 Ecuadorian children younger than 5 years of age. The nutritional status of the children was assessed by anthropometry and measurement of biochemical status indicators. The children were enrolled in a representative national nutrition and health survey. Antibody titers were analyzed as a function of the nutritional status of the children. For 12 of 14 antibody concentrations tested, underweight children showed lower antibody titers than did control children. The difference was statistically significant for antibody to both T-cell-dependent antigens (tetanus toxoid, rotavirus, respiratory syncytial virus) and T-cell-independent antigens (lipopolysaccharide, polyribosyl-ribitol phosphate, capsular polysaccharide). When children with a recent episode of diarrhea were excluded, many of the differences remained significant. When these children were further classified by age, only difference in titers of antibodies to respiratory syncytial virus and tetanus toxoid remained significant. No statistically significant difference was detected between underweight and control children with respect to protective antibody levels to four bacterial antigens. Anemic children showed significantly lower antibody levels to both T-cell-dependent and T-cell-independent antigens than did control children, and a higher proportion of anemic children had diphtheria antitoxin below a conservatively defined protective antibody level. No major differences in antibody titers were seen between children with different retinol and zinc concentrations in serum.