Hemoglobin (Hb), packed cell volume (PCV), red blood cells (RBCs), mean cell volume (MCV), and mean cell hemoglobin concentration (MCHC) were measured by a single operator in a single laboratory by means of an electronic particle counter in 942 healthy adults (491 females and 451 males) residing at five altitudes: 0, 1000, 1860, 2220, and 2670 meters above sea level. The subjects were carefully screened clinically, and subjects with low transferrin saturation (less than 15%) were excluded. In both sexes there was a differential behavior as a function of altitude, of Hb and PCV on the one hand, and number of RBCs on the other. The findings suggest the presence of two sequential mechanisms of adaptation to progressively lower atmosphere oxygen pressure: One operating from sea level to 1860 meters, which leads to a progressively increasing number of relatively microcytic RBC; and a second one -- from 1860 to 2670 meters -- in which there is an increased but constant number of progressively more normocytic RBC, so that a simplistic model of equal magnitude increases in the three parameters and is seen at 2670 meters, but not at the intermediate altitudes. The middle group's comparativity, essential to these interpretations, apparently was achieved with regard to time elapsed between sampling and testing and with regard to the people integrating the groups.