The new beta is labelled as build number 12D38, six builds newer than the first beta that was made available to those signed up to the AppleSeed program.
When it is released to the public the update is expected to bring updates to AirPlay, AirPort, Game Center, and the graphics drivers.
According to data released at the beginning of November, one in four Macs now run OS X Mountain Lion.
Apple is believed to be testing Mac OS X 10.9, its next iteration of its operating system, according to data spotted in analytics logs. The next Mac OS could be called Lynx, and is likely to arrive in 2013 with have Maps and Siri integration.
In November, Apple released Supplemental Update 2.0 for Mountain Lion 10.8.2.