Penicillin sulfoxide acid is obtained in higher, more economical yield and of surprisingly good purity from penicillin in fermentation filtered broth by concentrating the broth, diluting the concentrated broth with miscible organic liquid, optionally separating insolubles, treating the liquid mixture with a peracid oxidizing agent to form sulfoxide, adjusting the ph of the mixture to 1.5 to 2.5, and crystallizing the penicillin sulfoxide acid, with the aid of additional water, if necessary, and separating the solid penicillin sulfoxide.