Summary: The rosette test was used to study the morphology of antibody-forming cells obtained from lymphoid tissue of rainbow trout after the administration of sheep erythrocytes. Phase and electron microscopy revealed that five classes of cells were capable of forming rosettes: lymphocytes, plasma cells, blast-like cells, macrophages and cells resembling eosinophils. All these types of rosette-forming cells were found in the spleen and the anterior kidney. Although a quantitative relationship between serum agglutinins and numbers of rosette-forming cells was lacking, rosettes were numerous only in lymphoid tissue of animals that demonstrated minimal hemagglutinin titers of 1:64.