The kinetics of the hemolysis caused in human erythrocytes by Sendai virus is strongly dependent on the virus-to-erythrocyte ratio.Two processes appear to take place: a reversible one where the virus binds to the cell membrane, causes a lesion and eventually hemolysis, and is then slowly eluted in a still active form; and an irreversible one, possibly related to fusion of the viral envelope with the erythrocyte membrane, which results in the inactivation of the virus. The extent of hemolysis depends on the relative rates of the two processes, and the final number of lysed cells is approximately proportional to the logarithm of virus concentration. The kinetics of the process is sigmoid at high virus-to-erythrocyte ratios and hyperbolic under conditions where a virus particle causes the lysis of several cells before completion of the process.