The ability to conveniently detect single-base mutations in the DNA of clinical material without prior knowledge of the mutant sequence remains a diagnostic challenge. Most techniques suffer from a lack of general applicability to all DNA sequences, poor sensitivity, requirement for RNA samples rather than DNA, or necessity for performing DNA sequencing to uncover unknown point mutations. Recently, Montandon, et al. (8) described a novel method whereby segments of DNA amplified by the Polymerase Chain Reaction (PCR) can be rapidly screened for mutations through their formation of heteroduplexes with an end-labeled reference DNA followed by site-specific chemical cleavage at mispaired bases. Here we have applied this PCR-mismatch technique to a portion of the BCL-2 gene, using DNA samples derived from the biopsy specimens of patients with lymphomas or lymphocytic leukemias. The BCL-2 gene becomes activated through a t(14;18) chromosomal translocation in the majority of non-Hodgkin's lymphomas. Somatic point mutations were detected in the BCL-2 genes of 3 of 5 patient samples that contained a t(14;18). No mutations were observed for lymphomas lacking a t(14;18), nor in the DNA of over 20 normal persons. Further analysis excluded the possibility that the detected mutations represented hereditary polymorphisms or PCR-artifacts. Based on comparisons with direct DNA sequencing, the PCR-mismatch technique appeared to be both highly specific and sensitive. The possible mechanisms responsible for these somatic mutations in translocated BCL-2 genes and their functional significance are discussed.