Seminested polymerase chain reaction (PCR) was used to amplify the DNA fragments of the complementarity-determining region 3 of the immunoglobulin (Ig) gene heavy chain from the malignant cell specimens of patients with leukemias and lymphomas of B-cell lineage. Two different pairs of primers were used sequentially. Twenty of the 27 (74%) acute lymphoblastic leukemia (ALL) patients, 14 of 19 (74%) chronic lymphocytic leukemia (CLL) patients and eight of 20 (40%) non-Hodgkin's lymphoma (NHL) patients, who had rearrangement of the Ig gene heavy chain by Southern analysis, were positive by the seminested PCR. False-negative results appeared to occur more commonly in cases of lymphoma. The PCR analysis was also less likely to be positive if one-stage PCR studies with either pair of primers were both negative. The seminested PCR technique was found to have a high sensitivity of detecting malignant cells at the level of 0.02%. The clinical application of this assay needs to be investigated further.