The early described procedure for quantitative determination of oxacillin with the spectrophotometric method was used for the assay oxacillin tablets, capsules and injection preparations. It was shown that the excipients used in tablets and capsules did not interfere with oxacillin determination. The reproducibility of the method is +/- 1.5--2 per cent. The results of oxacillin determination with the new method and the currently used methods were compared.