A decrease in intra-platelet 5-hydroxytryptamine (5-HT) may be a possible marker for primary thrombocytosis and there are several different methods for the measurement of platelet 5-HT content (e.g. fluorimetry, high-performance liquid chromatography and radioimmunoassay). The present study has shown that, in laboratories with radioprotected areas, radioimmunoassay of 5-HT is a simple method which correlates well with a standard fluorimetric technique and yields satisfactory results in patients with thrombocytosis.