The human pro-monocytic leukemia U937 cell line was previously reported to become rho degrees cells after a long-term ethidium bromide exposure. In the authors' extensive PCR studies with different pairs of primers for the mtDNA molecule they showed that these U937-rho degrees cells, after being cultured in their laboratory for a time, did replete their mtDNA. That the cells grew well in the normal medium (RPMI 1640 plus 10% fetal calf serum and 2 mM l-glutamine) as the parental cells also suggests that these cells contain functional mitochondria and mtDNA molecules. Further experiments showed that the cells cultured in the medium with pyruvate and uridine rather rapidly and strongly adhered on the culture flask walls while the cells cultured in the medium without pyruvate and uridine either floated or only very loosely covered the culture flask walls. Ultrastructural examination showed that the floating cells, which were often found in the culture without pyruvate and uridine, were rather similar to monocytes in nature, like the original U937 cells, while the attached larger cells appearing earlier in the culture with pyruvate and uridine demonstrated macrophage differentiation, indicating a differentiation effect of pyruvate and uridine on the cells.