Summary: The utility of light scattering measurements on protein systems is discussed. Light scattering data for bovine serum albumin and its homologous rabbit antibody are presented. They describe the turbidity of the system as a function of time. It was found that the rate of aggregation of antibody and antigen molecules into large complexes is dependent on the antibody-antigen ratio. The existence of soluble aggregates in the antigen excess region is verified. Evidence is discussed which indicates that all reactions occurring in an antibody-antigen system are bimolecular.