We studied differences in chromatin patterns and the spatial localization of centromeres of chromosome 11 during the cell cycle between normal peripheral blood lymphocytes (PBL) and human promyelocytic leukemia cells (HL-60) using fluorescence in situ hybridization. The pericentromeres in both cells were located at the periphery during Gq (quiescent) phase, but moved towards the nuclear center in G1 and mid-S phase. During G2, the pericentromeres of PBL continued to move towards the nuclear center whereas those of HL-60 returned to the periphery. The angle defining the spatial location of two pericentromeres, in reference to the center of the nucleus, increased in PBL cells from a mean of 67Β°d uring Gq phase to 106Β°during G1 phase (P F 0.01), and the two pericentromeres remained wide apart throughout the entire cell cycle. In HL-60, the angle also increased during G1, but then decreased during mid-S and G2 phases. Both cells exhibited pericentromeric signals during Gq that were round and compact, and the entire chromatin was loosely condensed. The signal became more loose and dispersed during the G1 and mid-S phases. The pericentromere signal varied during G2 and was generally rod-like or bipartite with condensation of the entire chromatin or chromosome-like. Our results suggest that subtle but important differences in spatial localization of pericentromeres are present during the interphase between normal PBL and HL-60 cells.