Percentage of megathrombocytes, platelet regeneration time (PRT) and platelet-Associated IgG (Pl-A-IgG) were investigated in 12 patients with clinical features consistent with idiopathic thrombocytopenic purpura and in 11 patients with thrombocytopenia associated with chronic liver disease. Bone marrow smears were also examined and megakaryocytes classified into stages I–III according to the current principle. Of 12 patients with idiopathic thrombocytopenic purpura the percentage of megathrombocytes was increased in 9, PRT reduced in 10, and Pl-A-IgG increased in 8 patients. A statistically significant correlation was found between the percentage of megathrombocytes and the level of Pl-A-IgG. A slight correlation was also found between PRT and the percentage of megathrombocytes, while a significant correlation was found between megakaryocytes in stage I and the percentage of megathrombocytes, suggesting that growth of megakaryocytes probably contributes to platelet heterogeneity. In patients with thrombocytopenia and chronic liver disease, the percentage of megathrombocytes was in the normal range. A moderately increased level of Pl-A-IgG was found only in patients with active chronic hepatitis, PRT was reduced only in a few patients, while most of them also showed an increased level of Pl-A-IgG.