Ethidium bromide (EB) is known to intercalate between stacked base pairs without specific base-pair preference. Its use in cultured human lymphocytes and Burkitt's lymphoma cells resulted in the accumulation of cells in prophase and prometaphase stages. Inhibition of mitotic chromosome condensation as a possible mechanism involved in this phenomenon is discussed. A simple method for obtaining high-resolution banding patterns on elongated chromosomes was devised as follows: Human lymphocytes cultured for 3 days with phytohemagglutinin were exposed to EB (5-10 micrograms/ml) and Colcemid (0.02 micrograms/ml) simultaneously for 2 h and then routinely harvested for chromosome preparation. High-resolution G-bands were obtained by Giemsa staining following mild trypsin treatment.