Distribution of SIV envelope antigen and SIV RNA in small intestine. Representative images show (a-c) immunohistochemical staining for anti-SIV envelope (brown cell surface staining) and (d-f) in situ hybridisation for replicating SIV (purple stained cells). Low levels of anti-envelope staining within the lamina propria/villi of Group A animals (a) corresponds to sites of low level viral replication (d, arrows). Higher levels of anti-envelope staining within the follicular marginal zones of Group B animals (b) and follicles of Group C animals (c) also correspond to sites of increased levels of viral replication (e, arrows; f, within oval). Magnification ×100.