Pure penicillin acid (I) is made to react with a P halide to form an acid halide of I. Thus, 5.4 g. PCl3 and 27 g. CHCl3 were mixed with K penicillin G, dry pyridine added, kept at 5.degree. for 48 hrs., pptd. with abs. Et2O, filtered, and the filtrate treated with petr. ether. The acid chloride of I thus obtained can be recrystd. to give a white, nonhygroscopic powder, sol. in org. solvents and insol. in aliphatic hydrocarbons. It loses its activity slowly. The acid bromide and acid iodide of I are prepd. in a similar manner.