Chromosomes have been studied in cultures of nucleated peripheral blood cells in 30 patients with myeloma. Both unstimulated cultures and cultures stimulated by phytohemagglutinin (PHA) were examined in all cases. Mitoses were observed in 13 unstimulated cultures of peripheral blood from 12 cases, providing evidence for the presence of circulating abnormal cells in these patients. In 19 stimulated cultures of peripheral blood from 17 cases, abnormal mitoses were present, seven of these included abnormalities involving a chromosome of group 13-15. In five cases, a large acrocentric chromosome was observed similar to that previously described in some cases of myeloma. In eight cases, chromosome studies were also carried out on bone marrow to allow comparison of methods. Examination of stimulated peripheral blood cells was found to be the better method. Marked aneuploidy was not a feature of the results. No correlation was found between the cytogenetic abnormalities and previous treatment, disease stage, further progression of the disease, or serum and urinary protein abnormalities. The results are discussed, particularly with reference to technical factors influencing the frequency of demonstrable chromosome abnormalities in these cases and the relative merits of examination of peripheral blood and bone marrow.