The effects of butoxyacetic acid (BAA), a metabolite of the important solvent 2-butoxyethanol, on rat and human red blood cells (RBCs) were investigated. Rat RBCs demonstrated decreased deformability as assessed by a sensitive polycarbonate sieve filtration technique and an increased mean cellular volume after incubation with 0.2 and 2.0 mM BAA for 1-4 h. Evaluation of erythrocyte morphology showed that rat RBCs exposed to BAA became spherocytic and lysed to form erythrocyte membrane ghosts. Hemolysis of rat erythrocytes was rapid in 2.0 mM BAA. Changes in the deformability of rat erythrocytes appear to precede hemolysis. Treated rat erythrocytes also demonstrated a tendency to agglutinate and to release hemoglobin, which formed visible precipitates. None of the adverse effects seen in the RBCs of rats were observed in human blood similarly incubated with 2.0 mM BAA.