The average glycosaminoglycan content in control spleens, expressed as uronic acid, was 0.23 +/- 0.02 mg/g of dry wt; the average glycosaminoglycans content in spleens of CML patients, expressed as uronic acid, was 0.91 +/- 0.23 mg/g of dry wt. In control and in leukemic spleens the same glycosaminoglycans were present, that is hyaluronic acid, heparan sulphate, dermatan sulphate, chondroitin-4-sulphate and chondroitin-6-sulphate. However, in leukemic spleens the normal quantitative relationship between these glycosaminoglycans was greatly modified; in fact in control spleens hyaluronic acid, heparan sulphate and the chondroitin sulphates were present in almost equal proportions, whereas in leukemic spleens the chondroitin sulphate group alone represented almost 9/10 of all the glycosaminoglycans. Since this proportion is weakly modified in leukemic spleens in which the number of myeloid cells has been notably reduced after chemotherapy, we may suppose that this phenomenon is due to the very marked modifications which take place in the micro-environment of the leukemic spleen.