Sixteen cases of gastrointestinal B- and T-cell lymphoma were investigated for clonal rearrangements of immunoglobulin and T-cell receptor genes. In all cases the immunogenotype data corresponded well with the immunohistochemical findings, thus confirming the biologic heterogeneity of these lymphomas. Some cases could be distinguished by additional heterogeneity in their genotype which could not be detected by immunohistochemical methods. Moreover, the B-cell lymphomas were studied for the presence of the chromosomal translocations t(11;14) and t(14;18) using probes for bcl-1 and bcl-2 genes, respectively. While bcl-1 rearrangement was not present in these gastrointestinal lymphomas, one case of diffuse low-grade lymphoma of mucosa-associated lymphoid tissue and three cases of centroblastic lymphoma (diffuse large cell) could be shown to have detectable bcl-2 rearrangement.