Abstract.Quantification of platelet‐bound immunoglobulin is widely used in the evaluation of thrombocytopenia. Several methods have been devised among which labelled ligand‐binding assays seem to be most appropriate. In series of adult patients such assays have been shown to be superior in separating immune‐thrombocytopenia from thrombocytopenia of non‐immune causes. We studied 62 children with thrombocytopenia of various causes, using radiolabelled protein A as a ligand to measure platelet‐surface bound IgG. The test was highly sensitive (93%) in detecting immune‐thrombocytopenia. The specificity, however, was only 57 %, which is less than in published studies of adults. In a number of cases presumed to be non‐immune‐thrombocytopenia, notably a few patients with leukaemia and bone marrow aplasia, we found increased amounts of platelet surface‐bound IgG. The significance of this finding is not clear. An indirect assay measuring platelet‐specific IgG in plasma was less sensitive (46 %) but highly specific for immune‐thrombocytopenia (89 %). The measurements of platelet‐surface‐bound IgG and platelet‐specific IgG in plasma are of limited diagnostic value in childhood thrombocytopenia but are useful in following t