The presence of an unidentified tumor suppressor gene on the long arm of chromosome 13 which could be involved in the development of B cell chronic lymphocytic leukemia has been suspected because of frequent deletions of the locus D13S25 which lies 1.6 cM telomeric to the retinoblastoma gene. In order to accurately map this gene, cells from 25 B cell chronic lymphocytic leukemia tumors have been analyzed for allelic loss using a panel of microsatellite markers located in this region. These markers, which stretch from the retinoblastoma gene to the Wilson disease gene, have been ordered for their rank from centromere to telomere. In addition to the data obtained from deletion pattern of these markers, results from preliminary pulse-field electrophoresis studies enable us to redefine the minimal deleted area from more than 1 cM to 280 kilobase around D13S25.