The human and murine CD21 gene products have been functionally linked to B cell activation by the co-ligation of the BCR and the CD21/CD19/CD81 complexes. Binding of low levels of antigen complexed to the complement ligand(s) for CD21 enhances B cell activation compared to the stimulation caused by antigen alone. Mice lacking functional CD21 predispose to autoimmune responses suggesting that this receptor may also play a negative role: thus in the presence of excess complement-bearing immune complexes, B cell antigen-specific activation may be inhibited. This possibility was investigated using intracellular calcium elicitation analyses to follow BCR-mediated activation. Ligation of the BCR and limiting quantities of the CD21 receptor demonstrated the expected enhanced cellular response compared to BCR ligation alone: CD21 ligation alone demonstrated no alteration in calcium flux. However, co-ligation of the BCR with excess CD21 binding resulted in the elimination of the calcium response, suggesting that CD21 ligation was down-modulating the BCR response. Immunoprecipitation of kinases associated with the BCR and CD21/CD19/CD81 complexes demonstrated that Lyn is preferentially depleted from the BCR complex following excess binding of CD21. Localization of other kinases integral for B cell activation is not altered. These data suggest that excess CD21 ligand binding can negatively impact B cell activation by sequestering Lyn kinase away from the BCR complex.