Mercedes is out testing an S400 hybrid (not BlueTEC) version of its S-Class sedan in Houston, Texas, according to what appears to be some sharp spy photos from an attentive AutoblogGreen reader. As the site previously reported at the car's debut last fall, the S400 hybrid employs the company's 3.5L gasoline-powered V-6 and has a combined output of 299 hp with the electric motor assist.

Previously, it was reported that this car would remain in Europe; given that Mercedes is already pushing clean diesels here with the new line of BlueTECs, it's interesting to see that they're also testing the hybrid version stateside. The report said that the S400 will use a mild hybrid system created with BMW that sandwiches a 15kW electric motor between the transmission and the 3.5L gas V-6. The Mercedes mild hybrid system is likely to be the first mass production hybrid with a lithium ion battery, and should achieve about 30 mpg on average according to Mercedes.