The pathologic findings in 1,390 consecutive patients who had bone marrow examinations at Nashville Veterans Administration Hospital from 1977 through 1979 were reviewed. Seventy-three patients who did not meet diagnostic criteria for a small-cell lymphoid neoplasm (SCLN) and 11 patients with SCLN had, on marrow particle preparations: (1) at least three lymphoid aggregates and (2) suitable material available for immunoperoxidase studies with monoclonal antibodies UCHL-1 (CD45RO, pan T cell) and L-26 (CD20, pan B cell). Staining with UCHL-1 was difficult to interpret due to high background positivity in myeloid elements. With L-26, three distinct patterns of lymphocyte marking were identified within aggregates: (1) homogeneous--uniform marking of almost all lymphocytes; (2) mixed--even distribution of marking and nonmarking lymphocytes; (3) and focal homogeneous--collections of uniformly marking lymphocytes either surrounding or surrounded by nonmarking lymphocytes. A homogeneous marking pattern was the predominant pattern in 8 of 11 patients (73%) with SCLN. Only 6 of 73 patients without overt SCLN marked in a homogeneous pattern, and these were always associated with aggregates with other staining patterns. All patients with apparently non-neoplastic lymphoid infiltrates had mixed (67 of 73) or focal homogeneous (32 of 73) patterns of aggregate marking, whereas only 5 of 11 patients (45%) with extramarrow SCLN had aggregates with these patterns. The size and number of aggregates with a homogeneous marking pattern further helped discriminate between the patients with SCLN and those with apparently non-neoplastic lymphoid aggregates. These findings suggest that a homogeneous pattern of lymphoid aggregate staining with L-26 is more common in patients with SCLN than in those patients with lymphoid aggregates and no evidence of neoplasia. Paraffin immunoperoxidase staining with L-26 may be a helpful adjunct to histopathologic examination in evaluating marrow lymphoid aggregates.