A method of evaluation of platelet bound IgG (PA IgG) is described. This method rely upon the fixation of a radio-labelled goat anti-human IgG on platelets. Each new batch of labelling of the antiglobulin is tested with a reference system using a semi-purified anti-D IgG and O R1R1 and O rr erythrocytes. This system is used for the standardization and the validation of the antiglobulin, and it allows to make correspond an amount of antiglobulin to a known quantity of IgG molecules. The normal level of PA IgG in 98 healthy controls was found to be 351 +/- 252 antiglobulin combining sites (ACS) per platelet. The upper limit (99% confident limit) in the controls is 999 ACS per platelet. 412 thrombocytopenic patients have been tested, 78.7% of ITP had an elevated PA IgG. However positive results are also frequent in other thrombocytopenia even when an immune mechanism cannot be evoked. These positive results raise the problem of the signification of PA IgG and of their exact nature. Further dates are necessary to ascertain the role of auto-antibodies, immune complexes or of platelet membrane modifications in PA IgG increase.