The glycoprotein alkaline leukocyte phosphatase (ALP) can be used as a marker of maturity in neutrophilic granulocytes as the activity of ALP increases during neutrophilic differentiation. Severe congenital neutropenia (SCN) or Kostmann's syndrome is a congenital disorder characterized by a maturation arrest of myeloid progenitor cells at the promyelocyte/myelocyte stage that can be treated with recombinant human granulocyte colony-stimulating factor (rhG-CSF). Until recently there have been no reports on ALP activity in neutrophils of patients suffering from SCN, despite the fact that ALP is an important correlate of the functional activity of normal neutrophils. Thus, we conducted experiments to assess ALP activity in neutrophils from eight SCN patients before initiation of rhG-CSF treatment and an additional 17 SCN patients already receiving rhG-CSF. All eight patients analyzed before initiation of rhG-CSF treatment showed ALP activity in their neutrophilic cells. Four patients had normal and four patients had elevated ALP levels. Of the 17 patients already on treatment with rhG-CSF, 15 patients showed elevated ALP activity levels, one patient had normal ALP levels, and one patient showed abnormally low ALP activity. Thus, with the exception of a single patient, we observed normal or elevated ALP activity in neutrophils from SCN patients. As described in studies on the effect of rhG-CSF on ALP activity in healthy individuals, ALP activity was higher in SCN patients already on treatment with rhG-CSF. Therefore, our results indicate that SCN patients are not deficient in ALP activity and suggest that ALP is not deregulated in the majority of these patients.