Neutrophils (PMN) from 20 patients with atopic disease (atopy), the parents of 8 of the patients and 10 normal controls were studied by light-microscopic cytochemistry. The results revealed that myeloperoxidase (MPO) and acid phosphatase (ACP) activities in PMN in all cases significantly decreased and alkaline phosphatase activity was normal. Parents or parent of 7 of those patients had PMN enzyme deficiency similar to that of the patients. The results indicated that a primary combined and partial deficiency of MPO and ACP in PMN azurophilic granule existed in atopy. It is postulated that the deficiency led to reduction of PMN bactericidal power and delay of bactericidal action. Foreign bodies which were partially degraded could possess antigenic property. This is believed to be the important cytobiological mechanism of the tendency toward infections and formation of sensitive antigen in atopy.