In 80 patients with different platelet counts serum and plasma potassium concentration was measured, 30 minutes after blood sampling and three and six hours of storage. Serum and plasma concentrations differed, depending on the platelet count. In reactive thrombocytosis the mean difference was 0.7 mmol/l, independently of the time since sampling. In myeloproliferative diseases and with platelet counts of more than 1000 X 10(9)/l is was time-dependent. The mean difference was between 0.8 and 1.8 mmol/l, in individual cases as much as 2.6 mmol/l. Plasma potassium levels were independent of time and platelet count. The possibility of pseudohyperkalaemia in serum must be kept in mind whenever the platelet count is increased, regardless of its cause. In case of doubt potassium must be measured in platelet-free plasma.