Cytogenetic studies were performed on lymph node biopsies from 60 patients with indolent (low grade) non-Hodgkin's lymphoma. Thirty-two of the 39 successfully cultured biopsies had abnormal clones. The 32 abnormal clones represented the following histologies: seven small lymphocytic lymphoma (SL), eight follicular small cleaved cell lymphoma (FSC), 14 follicular mixed, small cleaved, and large cell lymphoma (FM), and three composite lymphomas. One of the composite lymphomas had FSC/DSC (diffuse small cleaved cell) and the other two FM/DM (diffuse mixed, small cleaved and large cell). Twenty-seven of the 32 biopsies were immunologically typed, and all were B cell. The clones all exhibited more structural than numerical abnormalities, and there was no difference in the modal chromosome number of the abnormal clones found in each histology. Biopsies with no normal cells were more frequently found in the SL histology (71%) than in the two follicular lymphoma groups (54%-55%). A translocation of the 14q32 segment was the most common abnormality found in all three histologies. In the follicular lymphomas a t(14;18)(q32;q21) was seen in 52% (13 of 25) of these patients, this translocation was not observed in the SL patients. Overall 84% (21 of 25) of the follicular lymphoma patients had abnormalities of 14q32 and/or 18q21. Other specific abnormalities included anomalies of chromosome #3 in FM, an abnormal 10q in FSC and FM lymphoma, and a high incidence of +18 and chromosome #1 abnormalities in patients with t(14;18). The presence of specific chromosome abnormalities in the indolent lymphoma patients suggests a relationship between certain karyotypic features and histology.