The aim of the present study was to test the in vitro antimicrobial properties of a cytotoxic factor isolated from human leukocytes and for an effect on DNA synthesis. A microdilution broth method was used to determine MIC values for fungi and bacteria. Flow cytometry was used to examine the effect on DNA synthesis. The MIC values for fungi were in the range 1-128 microg CF/mL and concentrations 2-4 times the MIC were fungicidal. CF had no effect on bacteria (MIC and MBC values > 1024 microg CF/mL). Growth experiments performed with increasing concentration of CF showed a dose-dependent effect on growth of the yeasts tested. We confirmed that zinc ions could abrogate the antiproliferative effect of CF. Flow cytometric experiments showed that CF had no effect on the cell cyclus. The present study shows that this endogenous substance isolated from human leukocytes has growth inhibitory properties towards fungi. Because of the origin from human leukocytes, CF may have a role in the non-specific defense against fungal infections. The use of CF may potentially have a role in the treatment of infections caused by fungi. Further investigations to characterize CF chemically and to study the antifungal mechanism are necessary.