Helminthic infections can cause an IL-4-dependent polyclonal synthesis of IgE, which is the major cause of the elevated serum levels of this immunoglobulin in tropical populations. In the present study, we measured the IgE levels in the serum of children of different ages in slum areas of Caracas, Venezuela, where intestinal helminths are endemic. As would be expected, in children over 5 years of age, the IgE levels were highly elevated, and in addition, the cord blood serum levels were higher than those reported for populations where parasitic infections are uncommon. However, the IgE levels measured in the sera of children under 5 years of age were relatively low, and within the generally accepted normal ranges. This was surprising because the latter children were infected by parasites and had high IgG antibody levels against helminths. We found, however, that whereas older children had detectable IL-4 levels in their sera, the younger children did not. These results suggest that, despite infection of these slum children by common intestinal helminths, a polyclonal stimulation of IgE synthesis did not occur, possibly due to a lack of IL-4 production by an immature immune system.