Opsonized yeast is known to bind strongly to the marginal zones in frozen sections of rat spleen (Kumararatne, D. S. et al., Eur. J. Immunol. 1981. 11: 858). This study reports an analysis of the cells involved in this binding. Sheep red cells coated respectively with C3b, C3bi or C3d were used as indicator cells. These showed homogeneous binding of both C3b and C3bi to marginal zones and germinal centers. C3d-coated red cells bound in a uniform speckled pattern to marginal zones. They also bound to germinal centers and the small lymphocyte zones of the follicles. Selective depletion experiments were undertaken to show that binding to marginal zones was a property of the IgM+ and IgD- B cells characteristic of this area. Binding to germinal centers was attributable to follicular dendritic cells. The C3d receptors in follicles were shown to be on IgM+ and IgD+ small B lymphocytes.