In six infants, from two families. the umbilical cords were still attached at 3 weeks of age. Five of these developed severe local and disseminated infections from which four died. Two of these children were tested, and both, including the survivor, had defective neutrophil mobility; in the survivor this was improved in vitro and in vivo by ascorbic acid. It is suggested that a primary genetic defect of a contractile protein could explain the association. The sixth child, with delayed cord separation but normal neutrophil mobility and no excess of infections, who has survived without special treatments, also has mastocytosis, apparently inherited independently.