Cell lines (U-266, U-1957, U-1996 and U-2030) established from 4 patients with multiple myeloma (MM) were analyzed cytogenetically. The cell lines represent different stages in B-cell differentiation as evidenced by ultrastructural and functional characteristics. The karyotypic pattern in 3 newly established myeloma lines was studied after a few months in culture and compared to the old myeloma cell line U-266, which was examined after 6, 7 and 8 years of continuous cultivation. Frequency of progressive numerical and structural aberrations during long-term cultivation and their correlation with alterations in growth properties were addressed. We describe the presence of a high frequency of both numerical and structural chromosomal abnormalities in the cells of all 4 myeloma lines studied. Chromosomes often associated with structural abnormalities were 1, 3, 6, 12 and 14. A 14q + marker chromosome was detected in 2 of the 4 cell lines. The breakpoints on the chromosomes participating in structural aberrations in myeloma exhibit some correlation to chromosome sites at or close to locations of mapped oncogenes. No translocations of c-myc were found. These data were further supported by Southern blot analysis (unpublished data). The extent of numerical, but not structural, aberrations correlates with the differentiation stage of the myeloma lines in that the 2 mature lines U-266 and U-1957 were both near-diploid. Multiple progressive chromosomal changes have emerged in U-266 during a period of 8 years with development of independence of feeder cells and increased growth rate. However, capacity for production of complete Ig molecules has remained stable.