Cefazolin (Ancef, Kefzol) is one of a series of semisyntheticcephalosporins in which the C-3 acetoxy function has beenreplaced by a thiol-containing heterocycle—here, 5-methyl-2-thio-1,3,4-thiadiazole. It also contains the somewhatunusual tetrazolylacetyl acylating group.