Studies have been done to determine the levels of human urinary granulocyte colony-stimulating factor in Egyptian patients with active bilharziasis. Colony-stimulating factor levels were measured by a semi-solid tissue culture colony assay with murine bone marrow as the target cell source. The levels in urine from patients with bilharziasis (mean 118) were found to be significantly elevated above control values found in normal human urine (mean 72) derived from the same population. This is the first demonstration of an effect of parasitic infection in man on the granulocyte regulatory system, and opens the way for future studies in this area.