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THERE have been a number of investigations on the cause of bleeding in patients with high platelet counts, particularly in essential thrombocythaemia, polycythaemia and other myeloproliferative disorders. A variety of tests of platelet function has been used, but no clear-cut pattern of abnormality has been defined (Gunz, 1960).In previous work on six patients with haemorrhagic thrombocythaemia, Glass (1961) found that the bleeding time, platelet adhesiveness, 5-hydroxytryptamine content and Factor-V activity were abnormal and could be correlated with the platelet count. Cronberg, Nilsson and Gydell (1965) reported that platelet adhesiveness was reduced when the bleeding time was prolonged. In a thromboplastin generation system, Glass found that the platelets appeared to be more active than normal at low concentration and more inhibitory at high concentration. It was thought that all her findings could be explained by postulating a surface abnormality of thrombocythaemic platelets which reduced their cohesion, interfered with the adsorption of plasma components and increased the availability of platelet phospholipid.further the platelet abnormality in thrombocythaemia; and (b) to see whether platelets from patients with transitory rises in platelet count, due to causes other than myeloproliferative syndromes, gave different results. Where appropriate, some of the previous findings have been included in the present study, which presents data on 16 cases of thrombocythaemia and 16 of temporary thrombocytosis.The present investigation was an extension of Glass's work attempting: (a) to MATERIALS AND METHODS Platelet counts followed the method of Brecher and Cronkite (1950). Rather than counting the platelets in a predetermined area of the chamber as is commonly done, the ruled area containing 150-200 cells was usually determined, so that all platelet counts should have the same sampling error. Except for the material taken from Glass (1961)~ all the counts were done by one observer using the same set of counting chambers and pipettes, and in most cases the same microscope. Counts were obtained from 32 normal subjects as well as from the patients.Bleeding times were done by a modification of Ivy's technique (Ivy, Shapiro and Melnick, 193s). After adjusting the arm cuff to 40 mm.Hg three wounds were made on the flexor surface of the forearm in quick succession with a disposable blood lancet, the triangular blade of which measured 4 mm. in length and 2 mm. in width at the base. The wounds were blotted simultaneously every 3 0 seconds, until all three had stopped bleeding. Wounds which bled profusely from the start (assumed to have perforated veins) or did not bleed at all were disregarded. Two end points were considered : (a) the longest time taken to stop, and (b) the mean time for all three wounds.

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