The effect of specific immunoglobulin (Ig) on specific binding of antigen to cells has been studied in a model system consisting of nurine myeloma cells (MOPC 315), MOPC 315 serum, and DNP conjugates. MOPC 315 serum, which has IgA specific for DNP, specifically inhibited the binding of DNP conjugates to these cells. Using this model it was found that cells have a marked advantage over free specific Ig in binding multivalent antigen molecules and retaining them in a bound state. Cells were able to specifically bind multivalent antigen in the presence of a large excess of free specific Igm the kinetics of antigen binding to cells was slow, and prolongation of time of incubation increased the amount of specific binding. Both antihapten and anticarrier Ig augmented nonspecific binding of multivalent but not of univalent hapten to control cells. Furthermore, antihapten Ig at low concentration increased antigen binding to specific cells.