Binding and toxicity of boronated phenanthridinium analogues were studied in vitro using cultured human malignant glioma cells. The compounds, 5-ortho- (5-o-CP), 5-para- (5-p-CP), 5-nido- (5-n-CP) and 6-nido-carboranyl phenanthridinium (6-n-CP) showed varying toxic effects. The cells were exposed to the compounds for 2 or 24 h. The span between non-toxic and toxic concentrations seemed to be very narrow. 5-p-CP was the most toxic compound, causing total cell death at a concentration of 5 micrograms/ml cell culture medium. None of the compounds showed toxic effects at a concentration of 1 microgram/ml. Viable cells incubated with the compounds at this concentration showed a > 100-fold accumulation of boron. Only approximately 1/4 of this accumulation was found in cells permeabilized and inactivated with acetone. Fluorescent images of acetone-treated cells showed clear uptake of the compounds in the cell nucleus, as for ethidium bromide, while for viable cells binding to structures other than DNA was also observed. These results were confirmed by subcellular boron determination. All tested compounds intercalate into DNA, as was demonstrated in cell-free systems with calf thymus DNA. The hypothesis is that the compounds are trapped in the cellular membranes of viable cells because of their lipophilicity, before reaching nuclear DNA.