Measurement of the plasma concentration of glycocalicin, the extracellular portion of platelet glycoprotein Ib, should prove to be clinically useful in the investigation of causes of thrombocytopaenia and as an indirect method of determining platelet lifespan. We describe an immunoradiometric method for the measurement of glycocalicin in plasma using two murine monoclonal antibodies. The assay has good reproducibility with coefficients of intra- and inter-assay variation of 5.4% and 7.0% respectively, requires standard laboratory skills only and, after overnight coating of the wells, enables analysis of multiple samples in approximately 4 h. With citrated plasma the result of the assay does not appear to be affected by one cycle of freeze-thaw, nor was there any observed difference in result between platelet-poor plasma (PPP) prepared by centrifugation at 1100 g for 15 min and plasma that was confirmed to be platelet-free. The plasma concentrations of citrate and ethylenediaminetetraacetate were found to affect the result of the assay thus making it important that only samples with the same type and similar concentration of anticoagulant be compared and that standards should be diluted in glycocalicin-depleted plasma with the same concentration of these anticoagulants as the unknowns.