• Twenty-two ocular adnexal lymphoid infiltrates were analyzed by electron microscopy as well as immunologically and cytochemically. Five reactive polyclonal lesions were found to be preponderantly composed of small mature lymphocytes (presumably T cells) with clumped nuclear chromatin, sparse cytoplasmic organelles, and numerous monoribosomes. In 11 monoclonal B-cell lesions, both the 1-μm plastic sections examined by light microscopy and the electron micrographs disclosed immature cells, with more dispersed nuclear chromatin, prominent nucleoli, abundant cytoplasmic polyribosomes, and increased numbers of mitochondria and strands of endoplasmic reticulum (particularly in plasmacytoid lesions). The remaining six monoclonal B-cell lesions were composed of comparatively well-differentiated cells requiring electron microscopy to show somewhat more prominent nucleoli, slightly less dense clumping of the nuclear chromatin, increased numbers of mitochondria and short segments of rough-surfaced endoplasmic reticulum, and monoribosomes rather than polyribosomes. The importance of distinguishing this group of well-differentiated monoclonal lesions from the less well-differentiated ones was underscored by the results of the follow-up examinations, in that no evidence of extraorbital disease has been discovered in the former group, while a 50% incidence occurred in the latter.