A general improved procedure for preparation of a cosmid library based on the use of a pBLcosT vectorwas described. The vector was modified to contain 2 tandem Xcml sites and was digested with Xcml to yield 2 terminal 3 T overhangs, capable of ligation with the insert that contains 2 terminal complementary 3 A overhangs. The resultant ligation mixture was packaged and a cosmid library in Escherichia coli was established.