SummaryCFU‐GM clonal growth was examined in bone marrow from patients with myelodysplastic syndrome (MDS) in the presence of Ara‐C at 10−8mand 4 × 10−8mand the response compared to the effect on progenitors from normal subjects. Seventy‐three percent of marrow cultures from patients with MDS showed colony growth below the 95% confidence limits for normal subjects in the presence of 10−8mAra‐C. At 4 × 10−8mAra‐C 43% of marrow cultures had subnormal colony growth. These results suggest that CFU‐GM from patients with MDS may have a greater than normal