Among 16 leukemia patients with abnormalities of the short arm of chromosome 12 (12p) were found 5 patients with an increased number of marrow basophils and a special M2 cytological feature. This new correlation between 12p abnormalities and M2-baso phenotype is presented. The localisation of c-k ras 2 genes at the same 12p site suggests a possible mutation of this c-oncogene.