The centrifugal "spinner" blood film preparation technique was investigated with respect to the preparation of blood films with the retention of conventional erythrocyte morphology. It was determined that the initial spinning produced a monolayer blood film while the duration of spinning affected the separation of the red cells in the monolayer and altered their individual morphologic characteristics. The spinner preparation resulted in a slide with the total surface similar in morphologic characteristics to one thin slice, or cross-sectional area, across a typical wedge preparation. The spinning time in effect determined the position of the cross-section. In many series of slides from the same blood, spun at increasing time intervals, at least one slide in each series contained erythrocytes whose morphology matched the "best" area of the corresponding wedge slide. The effects of red cell concentration and plasma viscosity were varied experimentally, to assess independently the effect of each on monolayer formation and morphology. The results indicated that it was necessary to spin for a longer time to overcome the viscosity effects due to the red cell concentration and also those due to plasma. Two evaluation criteria, "separation" and "central pallor development," were shown to produce different spinning time estimates. These differences were more pronounced at higher blood viscosities than at lower blood viscosities.