One dimensional patterns of proteins from homogenates and four cellular fractions i. e., nuclear, mitochondrial, microsomal, and cytosolic from normal and leukemic lymphocytes were compared. Results obtained revealed that the neoplastic transformation of normal lymphocytes into CLL and ALL ones is associated with the expression of some novel proteins. Electrophoretically-specific nuclear protein of B-CLL lymphocytes with mol. wt of 38/39 kDa was used as immunogen to produce rabbit antiserum. It was observed that obtained antiserum crossreacted with 38/39 kDa antigen of nuclear fractions from CLL and ALL lymphocytes (15 of 16 studied), but not with any of normal ones. It was shown, by Western blot technique, that the expression of 38/39 kDa antigen is correlated with progression of B-CLL disease.