Two patients with Rh mosaicism were studied, one of whom had a myeloproliferative syndrome. The red cell mixture was found to be composed of 30% DCe/dce and 70% dce/dce populations. The separated cell components did not differ in any other marker system and their in vivo survival as well as their glycolytic enzyme content were identical. Rh quantitative and kinetic assays did not provide any evidence for monosomy in D(-) population. The data are discussed with reference to published cases of Rh mosaicism.