Incubation of a 0.5% suspension of washed, normal mouse erythrocytes with ferriprotoporphyrin IX (FP) at 37 degrees C and pH 7.4 caused potassium loss, swelling, increased susceptibility to hypotonic lysis, and finally hemolysis. Hemolysis was not inhibited by incubation in the dark, malonyldialdehyde was not produced, and various free radical scavengers had no effect on the hemolysis. Only the sulfhydryl compounds, cysteine, dithiothreitol, and mercaptoethanol protected erythrocytes from FP. Potassium loss reached 90% within 30 min of exposure to 5 microM FP. This amount of FP caused greater than 50% hemolysis within 2.5 h. Sucrose (0.1 M) completely prevented hemolysis but had no effect on potassium loss. Likewise, reducing the temperature from 37 to 25 degrees C greatly retarded hemolysis but had no effect on potassium loss. These observations indicate that FP impairs the erythrocyte's ability to maintain cation gradients and induces hemolysis by a colloid-osmotic mechanism.