We have evaluated the expression and distribution of the cellular apoptosis susceptibility (CAS) protein in normal lymphoid tissue and malignant lymphomas. CAS protein, the product of the CAS gene, is associated with microtubules and the mitotic spindle. Immunohistochemistry with an antibody to CAS shows many CAS-positive cells in normal tonsils. The majority of strongly CAS-positive cells were localized to the dark zone of the follicles, whereas the mantle zone and interfollicular areas were essentially negative. Double staining for CAS and Ki-67 revealed co-expression of the two proliferation markers in approximately 85 to 90% of the CAS-positive cells. Different subtypes of lymphomas exhibited varying patterns of CAS expression. Low-grade non-Hodgkin's lymphoma generally revealed weak staining with CAS, with 10 to 60% of all cells being positive. In contrast, highly malignant non-Hodgkin's lymphoma and malignant cells of Hodgkin's disease displayed very strong CAS positivity, with staining of up to 80% of the atypical cells. Overall, the staining pattern of CAS and Ki-67 was superimposable within a particular lymphoma subtype. However, in all lymphomas we observed a significant fraction of CAS-positive normal and malignant lymphocytes that were Ki-67 negative, probably because they were momentarily noncycling cells. We conclude that a high expression of CAS correlates with proliferation of normal and malignant lymphoid cells. The fact that detection of CAS protein identifies a higher portion of proliferating and malignant cells than Ki-67 warrants further evaluation of CAS protein as a marker with a diagnostic potential.