We performed an immunohistochemical study of follicular dendritic cells (FDC) in the follicular area of the conjunctiva-associated lymphoid tissue (CALT). Hartley guinea pigs were sensitized with a topical application of an emulsion of ovalbumin and Freund's complete adjuvant in the eye. They were divided into four groups. The control (group A) underwent no sensitization. The sensitized animals were studied at 1 week (group B1) or 2 weeks (group B2) after the sensitization. Additional sensitization at 1 week after the initial sensitization was also performed (group C). Histological methods included methylgreen pyronine staining, alpha-naphthylacetate esterase staining, and enzyme-antibody method against S-100 protein. The uptake of topically applied peroxidase-anti-peroxidase (PAP) in CALT was also examined histologically. In each group, positive reticular patterns by alphanaphthyl acetate esterase staining and immunoperoxidase staining with anti-S-100 protein antiserum were found in the CALT follicular area. The positively stained cells were found to be dendrite cells by immunoelectronmicroscopy. An uptake of PAP was found in the CALT follicular area, suggesting the function of trapping and retaining antigen-antibody complex by FDC. It was concluded that dendritic cells in the CALT follicular area were identified to be FDC.