Three radioimmunoassays for the measurement of beta-thromboglobulin are described. The standard method, using antiserum in solution, could be used to measure plasma concentrations of beta-thromboglobulin with the results available after 2-3 d. The use of a greater dilution of antiserum and tracer, with delayed addition of tracer, resulted in a more sensitive assay suitable for measuring b-thromboglobulin in urine. The use of a solid-couples antiserum under non-equilibrium conditions allowed the measurement of plasma levels of beta-thromboglobulin after an assay incubation time of 1 h. These three radioimmunoassay systems for beta-thromboglobulin cover the likely clinical requirements for the measurement of this platelet specific protein.