In situ hybridization of the pHul-c-src probe to metaphase cells from three normal donors and two leukemic patients showed significant labeling in the proximal region of the long arm of chromosome 20q, with modal peaks of grains consistently at band 20q11.2. A secondary peak of grains was detected in the region 20q13.2-qter, the localization of SRC suggested by previous in situ studies. The exact localization of SRC is important for understanding the del(20q) chromosomal abnormality in myeloid neoplasias. Chromosome in situ hybridization and genomic studies showed loss of one allele of SRC in two patients with the deletion (20q). These results differ from previously published findings and suggest heterogeneity of the breakpoint at 20q11.2 in interstitial deletions of 20q, which characterize myeloid disorders.