The aim of our study is to characterize the disturbance induced by repeated BCNU treatments in 12 human brain tumor cell lines in terms of their collective behavior. This collective behavior was characterized by means of the Delaunay triangulation and Voronoi mathematical paving techniques combined with the computer-assisted microscope analysis of Feulgen-stained nuclei. This methodology enabled growth to be characterized in terms of cell colony size and density. In addition to this colony pattern characterization, the DNA ploidy level was assessed by means of DNA histogram typing. The cell proliferation level was also determined. Ten astrocytic and two medulloblastoma cell lines treated weekly with BCNU were analyzed. Study of the cell colony architecture and cell proliferation revealed specific BCNU-induced modifications in connection with the origins of the cell lines, i.e. astrocytoma (AST), glioblastoma (GBM), or medulloblastoma (MED). The BCNU-induced effect on GBM (the more malignant of the cell lines) was very different in that proliferation was weakened, but the cell colony density increased after a latency phase. The decrease in cell colony density and cell proliferation of MED seems to indicate that they are more sensitive to BCNU than GBM, but relatively tolerant of this type of chemotherapy in comparison with AST.