The serum from twelve children with kwashiorkor was deficient in its ability to support lymphocyte transformation in vitro, whereas lymphocytes from these children responded to phytohaemagglutinin and al-ogeneic lymphocytes in a relatively normal manner when cultured in normal serum. This serum abnormality improved with therapy and could not be clearly correlated with the degree of malnutrition, the presence or absence of infection or other laboratory manifestations of kwashiorkor. These observations indicate that defective cellular immune reactions in kwashiorkor may be symptomatic of a lack of some humoral factor and do not necessarily reflect an intrinsic cellular defect.