Cell kinetic differences have been described between acute lymphoblastic leukemia with L1 and L2 morphology. We now report cytokinetic and DNA ploidy findings of the rare L3 B-cell leukemia/lymphoma. Flow cytometry analysis of nineteen samples was performed by simultaneous DNA-RNA staining with acridine orange. RNA and DNA indices and cell cycle distributions were calculated. The RNA-index of the G0/G1 cells was 17.9 +/- 8.7 and the number of cells in S phase and S + G2M were 21 +/- 10.6 and 28.0 +/- 13.9 percent respectively. DNA aneuploidy was found in 6/19 (31.6%) and in two cases multiple aneuploid cell lines were observed. DNA aneuploidy and multiple abnormal stemlines adversely affected survival (p less than 0.05), while kinetic parameters did not affect survival (p greater than 0.05). The cytokinetic data are significantly different (S phase and RNA-I; p less than 0.001) than previously reported for the L1 and L2 ALL. Abnormal DNA stemlines were found in cases with no detected cytogenetic abnormalities. This study confirms that L3 ALL is characterized by significantly increased proliferation and provides a means for a flow cytometric identification of this subtype as compared to L1 and L2 ALL.