The underlying principles governing the second stage of hemagglutination, as described in 1965, have been reevaluated. This reappraisal reaffirms that red cell aggregation is dependent on the potential energy barrier (λ) between erythrocytes. This in turn is dependent on the ionic strength and dielectric constant of the bulk medium. Hemagglutination by IgG and IgM antibody is a function of the effective length of these immunoglobulins and the λ of the cells in the reaction mixture. Agglutination of cells occurs when the λ-potential is below a critical level. The enhancing action of natural and synthetic polymers (e. g., bovine albumin) is to lower zeta by raising the dielectric constant of the bulk medium. Several other recent concepts are discussed and the major differences discussed in light of data available.