Serum antibody (Ab) of channel catfish, Ictalurus punctatus, is predominantly a tetrameric, approximately 700,000-dalton protein. This tetrameric Ab dissociates into 8 distinctive subpopulations, each composed of equimolar heavy and light chains, when in the presence of denaturing solvents. The present studies utilizing purified anti-DNP Abs obtained from catfish monitored for either short (1-2 months) or long (1-2 years) periods post-primary immunization with dinitrophenylated horse serum albumin showed no demonstrable differences in the ratio of the covalent Ab subpopulations. Although significant humoral Ab levels were sustained during the 1-2 year monitoring period, there were no significant changes in the binding affinities or heterogeneity of the induced Ab.