Diagnostic vitrectomy is often performed because of suspected infection or malignancy. Giemsa, Gram, and Papanicolaou stains are used routinely to identify the components in the vitreous. Immunocytochemical staining of cellular components of vitreous specimens has the potential to significantly increase the amount of useful information that can be gained from histopathologic study. Vitreous specimens from 14 patients undergoing diagnostic or therapeutic vitrectomy for infection, suspected primary intraocular lymphoma, or uveitis were examined by immunocytochemical staining using monoclonal antibodies specific for leukocyte subclass antigens and immunoglobulin. The three classes of disorders showed characteristic patterns of staining, which were useful in confirming microbiologic and clinical diagnoses. Infections showed more pronounced neutrophils and macrophages, primary intraocular lymphomas demonstrated light chain restriction of the malignant B lymphocytes, and uveitis was characterized by the predominance of T lymphocytes. The routine use of immunocytochemical staining is recommended to characterize cellular infiltrates and increase the diagnostic yield from vitrectomy specimens.