The recognition and identification of subtle chromosomal changes in leukemic cells has greatly been facilitated since the advent of high-resolution banding techniques. However efficient utilization of these methods is often hampered by the paucity of leukemic cells during clinical remission, the variability of cell cycle length and tissue culture conditions. Therefore the detection of minimal residual disease in acute lymphoblastic leukemia by cytogenetic methods requires a preselection of material to be examined. In this preliminary report analyzable metaphases could be obtained in cultured cells from a colony assay for malignant peripheral B cell progenitors, whereas in marrow samples direct or 24 hours G-banding technique had failed to reveal metaphases in common Acute Lymphoblastic Leukemia patients during complete remission. It is believed that in common Acute lymphoblastic Leukemia patients this B cell colony assay permits the clonal expansion of residual circulating cells linked to malignant clone that are not detectable by classic hematologic and cytologic methods. In addition, this culture procedure substantially improves the sensibility of cytogenetic approach to the study of minimal residual disease in acute lymphoblastic leukemia during complete remission.