Parallel tubular inclusions were found in peripheral blood lymphocytes from 18 patients with various hematologic disorders, primarily lymphoproliferative processes, and 1 apparently healthy individual. The inclusions varied in size from 1000 to 6000 A and were usually membrane bounded. The microtubule-like structures comprising the inclusions ranged in size from 150 to 300 A and were packed in wall-to-wall contact with each other. Dense amorphous material and small dark crystalloids were frequently noted in the inclusions. There appeared to be a spatial and structural relationship of the inclusions with the centriole. The highest percent of lymphocytes with inclusionss (greater than 90%) were found in a patient with a lympho-proliferative disorder in whom 95% of the peripheral blood lymphocytes typed as T cells by spontaneous rosette formation with sheep red blood cells. (Am J Pathol 78:59-70, 1975)