The usefulness of low-molecular-weight B-cell growth factor (LMW-BCGF) for routine cytogenetic study of B cell non-Hodgkin lymphomas (NHL) was evaluated. Eighteen B cell lymphoma specimens were cultured for 4 days in the presence or absence of LMW-BCGF. After culture, conventional cytogenetic analysis was carried out, and the Morphology Antibody Chromosomes (MAC) method was used to study the frequencies of mitotic cells in different lymphocyte subsets. Our results showed that in 6 of the 14 lymphomas with a chromosomally abnormal clone LMW-BCGF produced an increased frequency of abnormal mitosis. In eight cases, LMW-BCGF did not increase, or even decrease, the frequency of karyotypically abnormal mitoses. In these eight cases, the total number of mitosis was often increased, but the mitotic cells were shown to be T cells. LMW-BCGF seems to have a clearly favorable effect on the cytogenetics of some B cell NHL, but apparently it is not a radical improvement over the routine techniques now being used.