Mutation of the ras oncogenes is the most commonly detected molecular abnormality in acute myelogenous leukemia and myelodysplastic syndromes (MDS). This molecular event may either be acquired by different subclones or by all malignant cells. The availability of the ras p21 monoclonal antibody Y13 259 makes possible the direct study of the distribution of the ras gene product in human malignant cells. The bone marrow smears from 41 patients with MDS were analysed by two independent observers after treatment with MoAb Y13 259, biotinylated goat antirat IgG, streptavidin, peroxidase and staining with diaminobenzidine. A high proportion of strongly positive smears was found among patients with MDS. This positivity was found in 25% of refractory anemia, in 80% of the refractory anemias with excess of blasts, and in 90% of those in transformation, while all 7 cases with chronic myelomonocytic leukemia were found positive. The percentage of positivity may suggest that activation of ras oncogene in associated with disease progression.