Decreased deformability of acetylphenylhydrazine-treated erythrocytes, measured by in vitro filtration experiments, was found to parallel Heinz body formation. The decreased deformability seemed unrelated to methemoglobin formation, ATP depletion, red cell size, or membrane effect. Hemolysis of Heinz body erythrocytes occurred during filtration of severely damaged cells, in an all-or-none fashion. In mixtures, separation of untreated cells from Heinz body-containing erythrocytes could be accomplished by filtration. There may be a relationship of these filtration properties to intravascular, in vivo destruction of severely damaged cells in Heinz body anemias.