An iodometric method was developed for the quantitative determination of new semisynthetic penicillins, sodium (2S, 5R, 6R)-6-[(R)-2-(4-hydroxy-1, 5-naphthyridine-3-carboxamido-2-phenyl acetamido]-3, 3-dimethyl-7-oxo-4-thia-1-azabicyclo [3.2.0] heptane-2-carboxylate (PC-904, apalcillin sodium) and sodium (2S, 5R, 6R)-6-[(R)-2-(4-hydroxy-1, 5-naphthyridine-3-carboxamido)-2-(4-hydroxyphenyl) acetamido]-3, 3-dimethyl-7-oxo-4-thia-1-azabicyclo [3.2.0] heptane-2-carboxylate (PC-462). These antibiotics were hydrolyzed to form corresponding penicilloic acids which consumed iodine. Accuracy and precision of this method depended upon the pH at which their penicilloic acids reacted with iodine, and pH 1.5±0.2 was found to be optimum. One mole of PC-904, PC-462, ampicillin or amoxycillin consumed 9.7 gramatom of iodine. The analytical results obtained by the iodometric method were in good agreement with those obtained by bioassay, including some samples partially decomposed.