Normal guinea pig and human megakaryocytes in suspension were measured with an optical micrometer. The range of megakaryocyte diameters in both species was from 10 to about 65 micrometer. Approximately 20%-25% of megakaryocytes were smaLler than 20 micrometer in diameter and were mostly missed in past studies. However, virtually the entire population of megakaryocytes was larger than all but a very small percent of the other marrow cells. This size range and the existence of a visual threshold size between the megakaryocytes and nonmegakaryocytes were confirmed by flow cytometric analysis of fresh unfixed cells. On human bone marrow smears there was some flattening of all cell types, but the megakaryocytes were consistently at least minimally greater in size than almost all the nonmegakaryocytes. Normal marrow cells greater than 20 micrometer in diameter were always megakaryocytes. Cells 14–20 micrometer were still noticeably larger than the general marrow population; thus easily found, they could be examined for specific morphological criteria. Size, therefore, is a useful first criterion for the identification of megakaryocytes. The larger sizes of megakaryocytes were related to their greater DNA content per cell (polyploidy) compared to nonmegakaryocytes. The relationship between megakaryocyte size, ploidy, and maturation was examined by the simultaneous measurement for the first time of each of these parameters in the same cell. Maturation was quantitated by the new scheme based on the progressive changes in megakaryocytes nuclear configuration. Within each maturation stage the mean cell volume of guinea pig megakaryocytes doubled with each ploidy doubling. Within each ploidy group, the sizes of megakaryocytes increased with maturation stage. However, maturation and polyploidization appear to be linked; the data showed that 80% of the low ploidy (4N-8N) megakaryocytes were immature and that 95% of the platelet-shedding megakaryocytes were 16N-32N.