In the present study we have investigated the uptake of cationic thiazine dyes and of the anionic Eosin Y by red blood cells (RBCs). Blood smears were stained with Azure B-Eosin Y, Methylene Blue-Eosin Y, Thionin-Eosin Y and with the cationic and anionic dyes alone at varying concentrations. Dye content of erythrocytes was measured with a Vickers M 85a microdensitometer. Nuclear chromatin features of white blood cells were investigated with the IBAS 2000 image analyser. Azure B favoured Eosin Y uptake of RBCs remarkably, and vice versa. There was no clear indication of that type of molecular interaction which characterizes the generation of the colour purple on polyanions. Methylene Blue and Thionin left Eosin Y uptake unaffected, but contamination of the standard Azure B-Eosin Y stain with Methylene Blue obviously affected Azure B-Eosin Y uptake, probably by competition of Methylene Blue and Azure B binding in RBCs. Furthermore, Methylene Blue contamination of the standard stain increased the rate of error in image analysis of white blood cell nuclei due to variations of staining intensity. For cytophotometry and image analysis of blood smears the standard Azure B-Eosin Y stain is by far superior to the commercial stain which normally is contaminated with Methylene Blue and some of the lower azures.