In order to quantitate the magnitude of the normal and Philadelphia (Ph') chromosome-positive(+) progenitor cells for various research and clinical settings/studies, we have applied the highly sensitive polymerase chain reaction (PCR) for examining the cells contained in individual hematopoietic colonies for chimeric bcr/abl mRNA, a specific molecular marker for chronic myelogenous leukemia (CML). Thus, individual 14-day CFU-GM colonies, obtained by growth of bone marrow cells from CML patients were removed from methylcellulose cultures and total RNA from each colony was isolated. First-strand complementary DNAs (cDNA) corresponding to all mRNAs in the sample were obtained by using random hexamers in a reverse transcription (RT) reaction. cDNA then served as the substrate in the PCR. To ensure the integrity of the RNA extracted from each colony, beta-actin and bcr/abl cDNA sequences were amplified in the same reaction vessel. Using this method, we have examined the colonies grown from three CML patients and found that 5 out of 5, 9 out of 9 and 8 out of 9 colonies contained a bcr/abl transcript. This method is simple, highly sensitive and should facilitate studies comparing the expression of various oncogenes in normal and leukemic hematopoietic progenitor cells.