The cytotoxic effects of hopanoids, including bacteriohopane-32, 33, 34, 35-tetrol (Tetrol), bacteriohopane-32-ol (Monol), diploptene, diplopterol and acetylated Monol (AcO-Monol) isolated from Acetobacter aceti, were tested against two leukemia cell lines. Tetrol and Monol have been shown to be toxic to mouse L1210 and P388 compared to the other hopanoids. By measuring the ESR spectra of the spin labeled membranes of these cells, it was shown that the incorporation of Monol resulted in a decrease in the fluidity of the membranes. The decrease in membrane fluidity may correlate, in part, with the cytotoxicity of hopanoids against the two cell lines.