Over seventy variants of the blue-green algal toxin microcystin have been identified. The two microcystin variants LR and LA vary in only one amino group i.e., arginine for microcystin LR (m-LR) and alanine for microcystin LA (m-LA). In the literature to date, the general consensus has been that m-LR and m-LA should respond similarly to a range of water treatment processes. This is the case for ozonation and biodegradation by organisms colonizing granular activated carbon filters; there is negligible difference in the response to these processes between the two variants. However, the adsorption of m-LR onto activated carbon is significantly higher than that of m-LA. This result is surprising as m-LA has a lower molecular weight, and is more hydrophobic, factors that would be expected to favor the adsorption of this compound over m-LR. The effect is seen on both negatively and positively charged carbons, indicating that the difference between the variants is not caused by electrostatic interactions with the carbon surface. It is possible th at electrostatic repulsion between the adsorbed m-LA molecules, with a net charge of -2, is responsible for the low adsorption. Microcystin LR has a net charge of -1, and would therefore experience lower intermolecular repulsion in the adsorbed state. Includes 23 references, figures.