To assess the biologic relevance of the morphologic distinctions between subtypes of small noncleaved cell lymphomas (SNCL), ie, the sporadic Burkitt's type (sBT) and the non-Burkitt's type (nBT), we have examined the molecular organization of several lymphomagenic oncogenes (c-myc, bcl-1, bcl-2) and the potential pathogenetic contribution of the Epstein-Barr virus (EBV). Twenty-nine cases of SNCL, not associated with immunodeficiency syndromes, were reviewed and classified as sBT (18 cases) or nBT (11 cases) without knowledge of the clinical or molecular data. Southern blot analysis of 18 sBTs found 17 to contain c-myc rearrangements. Fifteen of these comigrated with an Ig heavy-chain gene segment, indicating t(8;14) translocation. Chromosome 8 breakpoints were clustered in the first exon and the first intron of the c-myc gene. Chromosome 14 breakpoints mapped to the JH locus in three tumors, the S mu locus in nine tumors, and the S alpha locus in the remaining three tumors. Cases involving the S alpha locus appeared to have a more rapid clinical course. All sBTs possessed germline bcl-2 and bcl-1 gene fragments. In contrast, Southern blot analysis of 11 nBTs found none with c-myc rearrangements. Rather, three of 10 evaluable nBTs had bcl-2 rearrangements. The remaining seven showed no evidence of involvement by any of the lymphoma-associated oncogene/breakpoint regions studied. EBV genome was detected in two sBTs and in one nBT, and thus was not a distinguishing feature. These results indicate that the subtle histologic differences that distinguish subcategories of SNCL are significant biologically and reflect distinct molecular mechanisms of lymphomagenesis. Furthermore, the data suggest that the nBTs comprise a heterogeneous group with respect to their molecular genetic composition and confirm the remarkable molecular genetic homogeneity of the sBT group.