Atypical lymphocytes from nine young adults with acute infectious mononucleosis (IM) were studied for morphologic, ultrastructural, cytochemical, and membrane surface marker characteristics. There was an absolute increase in T lymphocytes in the patients. Atypical lymphocytes accounted for 83%-96% of the lymphocyte population. These lymphocytes contained cytoplasmic inclusions which ranged in size from 1000 to 6000 A, were usually membrane bound, and consisted of parallel arrays of microtubulelike structures. The inclusions, which have been referred to as parallel tubular arrays (PTA), were found in 15%-75% of the lymphocytes from the IM patients. Ultrastructural cytochemical methods demonstrated acid phosphatase activity within many of the membrane-bound PTA. The function of the PTA is unknown. Since they were observed only in the lymphocytes which appeared to correspond to the atypical lymphocytes on light microscopy, the majority of which typed as T cells, there appears to be an association between PTA and T lymphocytes. It is possible that PTA identify a specific subset of T lymphocytes which is expanded in IM. Alternatively, PTA may be a transient finding in lymphocytes appearing only in certain biologic states of the cell such as during T-lymphocyte activation.