A new immunoassay separation technique was developed based on the use of antibody-polymer conjugates with reversible solubility characteristics. Antibody was coupled to sodium alginate with water-soluble carbodiimide. These conjugates could be used in place of unmodified antibody in various assay formats and removed from solution when desired by converting the polymer to an insoluble form through pH adjustment or addition of metal ions. An enzyme immunoassay for theophylline was used to demonstrate the usefulness of the new approach for both manual and partially automated procedures.