Using 3H-TdR labelling radioautography in vitro, labelling index (LI) of the bone marrow cells from patients with CML was estimated before and after treatment by indirubin, and compared with myleran. The results showed that LI of the bone marrow cells was prone to decline after treatment by indirubin or myleran, and it was most marked on the myelocyte and polychromatophilic erythroblast. The LI reduction was less by indirubin than myleran. It suggests that the inhibition of indirubin on the bone marrow cells be weaker than that of myleran. However, long-term administration of indirubin would inhibit not only the granulocytic series but also the red cell series to a certain degree.