In this paper we describe an immunoenzymatic assay based on a rat monoclonal antibody (Ram kappa) developed to determine mouse IgG concentration, which is widely used for samples obtained on purification processes, like supernatant waste and the content of IgG in the vaccine (rHBsAg). This assay involves the use of a rat antibody-horseradish peroxidase-conjugated for the revealing of the antigen-antibody reaction. The rat antibody was produced in cell culture using a dialysis tube (DT). The immunoassay was standardized following several concepts, such as specificity, precision, and linearity. The result obtained permitted us to replace the use of polyclonal antibodies to determine the kappa light chain mouse antibodies by a rat monoclonal antibody of high sensibility and reproducibility. The assay permitted a reliable measurement of murine kappa Ig up to 0.68 ng/ml and was capable of quantifying 6.25ng/ml. Due to the high frequency of the kappa light chain in mouse antibodies this system acquires a great application. (C) 2004 Elsevier Inc. All rights reserved.