To assess the potential of using flow cytometric analysis of platelet glycoprotein IIb in the detection of heterozygotes in Glanzmann's thrombasthenia (GT) we compared the fluorescence intensity of anti-CD41-labelled platelets of obligatory heterozygotes with that of normal subjects. This study showed that normal subjects had relative fluorescences occasionally overlapping with heterozygotes. Use of an independent factor, the mean platelet volume, as a correction factor for platelet size differences eliminated this overlap. This study indicates that it is possible to confidently and rapidly predict the carrier status of family members of GT sufferers, both type I and type II, by flow cytometric means.