WASHINGTON - BP Plc is making oil leaking into the Gulf of Mexico more toxic to marine life by mixing it with Nalco Holding Co.'s dispersants, an environmental group said.

Oil and Nalco's Corexit dispersants combined have higher toxicity than either substance alone, Richard Denison, a senior scientist at the Environmental Defense Fund, said today by telephone. Denison yesterday posted on his website an analysis based on technical product bulletins that Nalco provided to the U.S. Environmental Protection Agency.