A protein fraction was purified to homogeneity from the culture medium of the wall-less (slime) strain of Neurospora crassa (FGSC 1118), which proved to be identical with DNA-uptake-stimulating factor (designated DUSF), which has been described earlier [Schablik, M. and Szabó, G. (1981) FEMS Microbiol. Lett. 10, 395-397]. The quantity of DUSF is measured by the amount of [3H]DNA uptake by Neurospora cells at standard conditions. Its relative molecular mass was 230,000. It has an isoelectric point of pH 5.5. This protein consists of two identical subunits, relative molecular mass 110,000.