Purified L-cell colony stimulating factor (CSF) and rabbit anti-CSF serum were used to devise a radioimmunoassay for this factor. The CSF was radiolabelled with the aid of lactoperoxidase and precipitated by a double antibody technique. Addition of unlabelled CSF caused a dose-related displacement of the labelled tracer. Similar results were noted with conditioned media and murine serum. The assay required only 4 days for completion as compared with 7 days for the conventional agar gel bioassay. Moreover, the radioimmunoassay proved more sensitive and accurate than the bioassay. This technique should allow further exploration of the role of CSF in granulopoiesis.