We have performed double marker sieving experiments with molecules ranging from ca. 0.5-3 nm dia. in order to evaluate the size distribution of the channels formed by complement in resealed sheep erythrocyte ghosts. Evidence is presented that marker release through the channels reached equilibrium between the ghosts and the extracellular fluid in a period of 3 hr and that the channels are stable at 37 degrees C for this period of time. Under these experimental conditions we have observed a differential in the endpoint release of inositol and sucrose, which indicates that some of the ghosts carried channels measuring between 0.7 and 0.9 nm dia. No differential was observed between release of sucrose and raffinose (0.9 and 1.1 nm mol. dia., respectively). Comparisons between sucrose and inulin (0.9 and 3 nm mol. dia, respectively) showed a difference in marker release. Also, there was substantial release of inulin, indicating the presence of channels above 3 nm in dia. Hence, the present data indicate formation of channels in three size ranges, namely, 0.7-0.9 nm, 0.9-3 nm and greater than 3 nm.