Content of actin and vimentin and organization of actin were studied in peripheral blood lymphocytes from healthy donors, circulating lymphocytes from patients with chronic lymphocytic leukemia (CLL) and lymphoblasts from children with common acute lymphoblastic leukemia (c-ALL). Actin and vimentin were quantified as percentage of total proteins by densitometric scanning of SDS-polyacrylamide gels loaded with Triton-X-100 cell lysates. In addition, the amounts of F- and G-actin were evaluated by ultracentrifugation of total lysates prior to electrophoresis. Compared to peripheral blood lymphocytes from healthy donors, CLL lymphocytes, but not blasts expressing c-ALL antigen, show a decrease in total actin and vimentin contents. Analysis of SDS gels after ultracentrifugation shows that F-actin is diminished of about 50% in both CLL and ALL cells compared to peripheral lymphocytes from healthy donors. The changes in actin and vimentin content and of actin organization may account for some of the functional abnormalities observed in leukemic cell populations.