The aggregation state of detergent-extracted cytochrome b5 was examined by gel filtration and sedimentation equilibrium ultracentrifugation. Both techniques indicated the presence of a mixture of monomer and octomer. The proportion of monomer was decreased as the buffer salt concentration was raised and was approximately 1 muM in 10 mM Tris acetate/0.1 mM EDTA. The monomeric cytochrome b5 was isolated by gel filtration and showed a decreased tendency to aggregate in dilute buffer, although its other physical properties were identical to those of the original cytochrome b5. The monomer was found to have a Stokes radius of approximately 26 A, as determined by classical gel filtration and by an equilibrium saturation method where the sample was monitored in the gel by measuring the absorbance of the gel column directly in a dual wavelength spectrophotometer. The gel filtration and ultracentrifugation data together with the studied on the monomeric cytochrome b5 suggest the octomer is not a rapid equilibrium with monomer and this observation should be taken into account in lipid binding experiments with cytochrome b5.