SummaryPlatelet‐associated Ig (PAIg) is raised in patients with idiopathic thrombocytopenic purpura. However, it is also raised in conditions not traditionally considered to be immune mediated. The latter finding suggests that raised PAIg may be an epiphenomenon of thrombocytopenia. The assays currently available do not distinguish between PAIg present due to a specific Fab binding or present due to non‐specific binding of Ig via Fc or complement receptors. In this paper we show that by prior incubation with normal rabbit serum, an assay for PAIg can be made specific for Fab bind