BP has restarted operations to stream dispersants directly into the main Gulf of Mexico oil leak despite fears the chemicals could themselves be harmful to the environment.
A mile-long tube was fed down to the leaking pipe on the sea floor and directly shot the dispersant into the flow, guided by remotely-operated robotic submarines.
State and federal agencies "consented to the third test today of subsea dispersant," BP spokesman John Curry told AFP.
The test began at 4:30 am (1500IST) "and will continue for 24 hours. After the test is concluded, further evaluations will be conducted," said Curry.
The dispersant is meant to break down the oil so that, over time, the slick is reduced to smaller particles that biodegrade instead of being left as chunky, thick globs that can choke both wildlife and vegetation.
Critics however say the dispersant causes just as many problems as it solves, and affects undersea life from the smallest microorganism on up.
"We are continuing to deploy dispersant at the seabed. It seems to be having a significant impact," said BP CEO Tony Hayward.
"We have an armada of ships, as you know, on the surface engaged in major skimming activity. That is proving to be pretty effective. We've skimmed of the order of 100,000 barrels of oily water," Hayward said.
"And we have an air force of planes deploying dispersants, and we're also conducting, as you know, controlled burns, and that -- the activity on the surface is going a very long way to containing the spill in the far offshore."
Louisiana State University scientists will study underwater and surface samples of the dispersant impact on the oily water mixture, Curry said.
Meanwhile, US Environmental Protection Agency administrator Lisa Jackson traveled to the gulf region on a two-day visit "to oversee efforts to mitigate the environmental and human health impact of the ongoing BP oil spill," the EPA said in a statement.
Jackson will be seeking "a thorough scientific assessment" of the spill's impact on the region, and will meet with scientists, "to discuss the potential impact of the use of dispersants on the spill on and below the surface of the water," among other things.
Environmentalists, shrimpers and fishers worry that the dispersant will kill fish and other organisms in the seafood chain.