Resting cells of Streptomyces clavuligerus NP-1, which possess deacetoxycephalosporin C synthase activity, have been shown previously to perform oxidative ring expansion of penicillin G in the presence of iron, ascorbic acid, and alpha-ketoglutaric acid to form deacetoxycephalosporin G. Further studies on this bioconversion indicated that use of MOPS or HEPES buffer at pH 6.5 more than doubled the extent of the reaction observed with the previously used Tris-HCl at pH 7.4. Levels of bioconversion as high as 16.5% were achieved at low penicillin G concentrations. Previously, conversion yields were < 1%.