Patients suffering from B-ALL were analyzed for residual leukemic cells by means of the PCR technique. Using primers specific for conserved domains of each VH-family and a primer universal to the JH-regions, immunoglobulin gene fragments have been amplified from bone marrow aspirate-derived DNA in six cases of B-ALL. Cloning and sequencing of the amplified fragments revealed the usage of the VH251-family in two cases. VH-families 1 and 3 were used in one case each. One patient showed abnormal rearrangement with the participation of two JH regions. In the remaining case a VH6 germline fragment was involved in an apparently abnormal rearrangement. Based on the sequence information, clone-specific DIG-labeled probes were designed and used in subsequent nonradioactive hybridization protocols to estimate the prevalence of residual leukemic cells in the course of therapy. Five of the six cases of B-ALL could be included in the rest cell analysis. In all patients investigated, residual leukemic cells were detected, independent of whether a normal or apparently abnormal rearrangement has occurred. These findings imply that the prevalence of residual leukemic cells in B-ALL indicates a high risk of relapse.