Cellular and humoral components of leukotaxis were studied serially in four male infants with severe combined immunodeficiency disease. Two of the four, both lacking B and T cells initially, had a significant defect in neutrophil and monocyte chemotaxis. The other two, who had a high number of immunoglobulin-bearing cells (B cells), did not have these cellular abnormalities. It contrast, defective generation of chemotactic factor following endotoxin activation was observed in all patients. The defects were corrected coincident with or soon after successful engraftment of either bone marrow or fetal tissues. The reported deficiencies may be another manifestation of the heterogeneity in SCID.