The bactericidal activity of monocyte-rich cell suspensions obtained from the peripheral blood of patients with fatal granulomatous disease (FGD), their mothers and normal controls was studied. Defective killing of Staphylococcus aureus, Strain 502A, by monocyte-rich cell suspensions from children with FGD was found when compared with monocyte-rich suspensions from normal controls. This defect, less severe, was also observed in the mothers of affected patients. In this selected population of mothers, the monocyte defect varied in parallel with the previously described neutrophil defect.