The evolution of immunoglobulin receptors at the surface of spleen rabbit lymphocytes during the primary response to sheep red blood cells (SRBC) was investigated. Using inhibition of rosette‐forming cells (RFC) by specific anti‐rabbit immunoglobulin sera and a combined ultrastructural autoradiographic and immunocytochemical method, two classes of immunoglobulins have been found on the surface of the same cell during the early phase of primary response to SRBC. Anti‐immunoglobulin sera did not show any additive effects on inhibition of non‐immunized and 7‐day RFC. Using a combined ultrastructural method, cells never displayed more than one label on their surfaces.