The human monoclonal antibody JDB1 was examined for reactivity to five breast cancer cell lines as well as normal fibroblasts. The JDB1 clone secretes both IgG3 and IgM antibody. In these studies both the total antibody (containing IgG3 and IgM) as well as the purified IgG3 and IgM fractions were examined for tumor cell binding reactivity. Peroxidase staining was observed in the breast cancer lines SW527, MCF-7, T47D, SKBR3, and MDAMB231, while GM179 and GM5758 fibroblast lines were negative for antibody binding. Tumor cells were examined using two techniques: a drop cell technique in which cells were fixed onto slides and also a cover slip assay in which cells were grown onto sterile cover slips and subsequently stained with the human monoclonal antibody using a peroxidase technique. Both cytoplasmic and membrane staining were observed for all of the breast tumor cells tested using both assays. In addition, a cellular ELISA was developed and used to quantitate the binding of these antibodies to tumor cells.