The granulocytes which are distributed by blood circulation close to the extern and intern body surfaces as well as in all organs protect the organism from microbial perils by phagocytosis and intracellular killing of bacteria and fungi so constituting a very important component of the granulocytic functions seriously impairs the host resistance and may entail a state of persisting infectious disease. As chronic mucocutaneous candidosis (CMCC) represents such a persistent generalized infection, we studied in vitro several functional activities of the granulocytes of 5 patients suffering from CMCC. 2 of them presented a non-familiar type of CMCC, the remaining 3 patients (father and 2 daughters) were subject to the hereditary type of CMCC. For comparison, by the same way we investigated the granulocytic functions of 51 clinically and immunologically healthy adult persons. Each of our 5 patients exhibited a reduced ability of the granulocytes to phagocytize and kill Candida albicans. In the CMCC family, the father had a marked deficiency of the oxidase activity of the granulocytes whereas in his daughters, the oxidase deficiency proved to be of a minor grade. In the sera of both daughters a phagocytosis inhibiting factor could be assumed to exist in addition to the granulocytic abnormalities. When heat-inactivated Candida albicans cells, however, were used for experiments, the granulocytes of each patient were able to phagocytize the germs at the same rate as did the granulocytes taken from the controls. With regard to alterations of the T cell function previously reported in CMCC, in all patients we also could demonstrate various symptoms of a T cell-dependent immunodeficiency. The results of the present in vitro-experiments furnish good evidence of a state of fundamental deficiency of the microphages in patients with CMCC, which may be the dominating cellular factor in the aetiopathogenesis of CMCC.