The Reagan administration yesterday gave the green light to an unusual joint venture of eight New England high-technology firms that have banded together to seek Defense Department contracts.

The action includes a blanket immunity from antitrust lawsuits.

Justice Department officials said this is the first time that they have granted such approval to a small-business joint research and development project. The department acted under an obscure section of the Small Business Act that has not been used before. One official called it "kind of a lost provision of the law."

The venture, which is known as Small Business Technology Groups Inc., was described by officials as precisely the kind of high-tech joint project that the administration is seeking to promote. Formed earlier this year, the Boston-based group was set up to scout for high-tech government contracts, primarily from the Pentagon, for its member firms. The central office will develop contract proposals, administer contracts once they are awarded and receive a share of the profits that then will be divided among the eight members.

Because the member firms have expertise in different and complementary areas, the intention is that SBTG members would create joint proposals for individual contracts, said Karen Hastie Williams, counsel for SBTG. Of the eight firms, none has more than 250 employes and some have fewer than 10.

In a prepared statement, Attorney General William French Smith said SBTG "illustrates the sort of private initiative which the government ought not to discourage. It poses no threat to competition in any market." He also repeated the administration's recent call for passage of broader legislation that would permit larger competing companies to establish joint research and development ventures without fear of antitrust suits.

The eight members of SBTG are Coverdale Associates Inc. of Newton, Mass., Data Signal Corporation of Newton, Mass., Expedition Electronics of Jericho, Vt., Geo-Centers Inc. of Newton Upper Falls, Mass., Millitech Corp. of Amherst, Mass., PACER Systems Inc. of Burlington, Mass., Spectral Sciences Inc. of Burlington, Mass., and Stratus Computer Inc. of Natick, Mass.