The antibody-forming cells which appear in the popliteal lymph node and efferent lymph of the sheep following immunization with boiled Salmonella have been studied by light and electron microscopy. Cells were incubated in monolayers with target erythrocytes sensitized with bacterial lipopolysaccharide. Three types of interaction between a proportion of the lymph cells and the erythrocytes surrounding them have been shown to indicate antibody formation: plaque-formation, immuno-cyto-adherence, and localized agglutination. At the peak of the response, 4 days after antigenic stimulation approximately 1 cell in every 200 from lymph node suspensions produces detectable specific antibody, while up to 1 cell in 20 in the lymph is active. For light microscope examination, individual antibody-forming cells were smeared in serum and stained with Leishman's stain. For electron microscopy, a number of active cells were clumped with antiserum to form a specimen of convenient size, then sectioned. Most of the active cells from efferent lymph are large and basophilic, while a small proportion are blastlike. These cells contain abundant free ribosomes and very little endoplasmic reticulum. In the node only, an additional class of antibody-forming plasma cells is found which have considerable amounts of endoplasmic reticulum in their cytoplasm.