Actin, a major cytoskeletal protein, was quantitated in normal and chronic lymphocytic leukemia lymphocytes. The actin content of normal human blood lymphocytes was 2.2 +/- 0.4 mg/10(9) cells and represented 6.6% +/- 1.8% of the total cellular protein. A significant decrease (p less than 0.001) was noted in chronic lymphocytic leukemia lymphocytes that contained 1.4 +/- 0.3 mg actin/10(9) cells, constituting 4.3% +/- 1.1% of the total protein. Normal T and B cells did not differ in actin content. Reduced actin levels were found in the T as well as in the B lymphocytes of “B-cell” chronic lymphocytic leukemia. The possible importance of the decreased actin level in the anomalous capping response and motility of chronic lymphocytic leukemia lymphocytes is discussed.

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