Microbeads for immunomagnetic bone marrow purging, to which sheep anti-mouse-IgG1(Fc) antibodies have been linked, and beads linked with antibodies against whole murine immunoglobulin were compared. Competitive binding studies, in which Fc fragments and Fab fragments were titrated onto the microbeads, followed by incubation with 125I-labeled whole mouse Ig, revealed that the beads linked with anti-mouse-IgG1(Fc) specifically bound the Fc region of the murine immunoglobulin molecules. The total amount of antibody of either IgG1 or IgG2 isotype that would adhere to microbeads linked with either type of antibody, as revealed by secondary binding with 125I rabbit antimouse Ig, was identical, suggesting that similar numbers of antibody binding sites were available. In cell depletion studies, it was found that if IgG1 isotype monoclonal antibodies were employed as binding intermediaries between the target cells and the microbeads, the efficiency of target cell depletion was superior with the anti-mouse-IgG1(Fc)-coated beads, even if the amount of MoAb coating the target cells was suboptimal. However, if the intermediary antibodies were of the IgG2 isotype, the efficiency of target cell depletion with these beads was inferior. These findings indicate that the efficiency of immunomagnetic bone marrow purging is dependent upon matching of the targeting MoAb and the secondary antibodies that link to the surface to the microbeads.