At Stanford he began to explore the field of bacterial plasmids, seeking to understand how the genes of plasmids could make bacteria resistant to antibiotics. At a conference on plasmids in 1972, he met Herbert W. Boyer and discovered that their interests and research were complementary. Plasmids were sent back and forth between Stanley Cohen, Annie C. Y. Chang, and others at Stanford, and Herbert Boyer and Robert B. Helling at the University of California, San Francisco. The Stanford researchers isolated the plasmids, and sent them to the San Francisco team, who cut them using the restriction enzyme EcoRI. The fragments were analyzed and sent back to Stanford, where Cohen's team joined them and introduced them into Escherichia coli. Both laboratories then isolated and analyzed the newly created recombinant plasmids.