We have recently described the potential use of fluorescence in situ hybridization (FISH) to detect tumor-specific rearrangements of the immunoglobulin heavy-chain (IgH) gene in interphase nuclei. Using yeast artificial chromosome (YAC) clone Y6 containing variable region (VH) gene and bacteriophage clones Ig gamma, we analyzed 70 patients with B-cell non-Hodgkin's lymphoma (NHL) and compared the results with those obtained by the conventional G-banding method. Tumor-specific rearrangements of the IgH gene equivalent to 14q32 translocations were defined as separate signals of VH and Ig gamma genes or those of Ig gamma genes and referred to split signals. Twenty-nine patients (41.4%) showed split signals. Among these, 13 did not show 14q32 translocations by G-banding: three with other chromosomal abnormalities, one with normal karyotype, and nine with no analyzable metaphases. The partner sites of 14q32 translocations were identified in 17 patients by FISH: t(3;14)(q27;q32) including a complex variant was observed in nine patients, t(14;18)(q32;q21) in four, t(8;14)(q24;q32) in three, t(14;19)(q32;q13) in one, and t(11;14)(q13;q32) in one. Six of nine patients with t(3;14) or its variant and one of three with t(8;14) were diagnosed as having respective translocations only by FISH. Translocation t(3;14) was found most commonly, and was correlated histologically with diffuse lymphoma with large-cell components. These results indicate that interphase FISH with IgH gene probes promises to be a rapid and reliable method for use in the diagnosis of B-cell NHL.