Simultaneous determination of trimethoprim and sulfamethoxazole in tablets and in injectable solutions were made by high-pressure liquid chromatographic method. A 30 cm μ Bondapak C 18 with the mobile phase 75 % methanol at a flow rate of 2.0 ml/min was used. Excellent linear rela-tionship between the peak heights and the concentrations were demonstrated by regression analysis.Comparison of the assay results in various samples by HPLC method and the B.P. method was also made to enssue the reliability of the new method. Trimethoprim and sulfamethoxazole preparation known as Co-trimoxazole in British Pharma-copoeia has been widely used in antimicrobial therapy. Because it is claimed that a potent synergism in bactericidal action is obtained by the two drugs that block two sequential steps in folinic acid synthesis.1 The only one official method available for the quantitative determination of trimethoprim and sulfamethoxazole preparations is B. P. method.2 The latter is subject to separate the two com-ponents by extraction and determine each component by spectroscopy and colorimetry. Some high-pressure liquid chromatographic method (HPLC) were reported for the determination of sulfa drugs,3,4,5,6 however, the HPLC determination of trimethoprim and sulfamethoxazole was not demonstrated yet.This report presents a simple and rapid HPLC method for simultaneous quantitative determination of trimethoprim and sulfamethoxazole in tablets and in parenteral solution. 高壓液體層析定量法，曾被利用於快速定量幾種磺胺劑，但對Trimethoprim及Sulfamethoxazole之測定，則尚未見發表。一般利用B. P.方法定量Trimethoprim及Sulfamethoxazole之複合製劑皆須經抽出分離及呈色反應，操作費時，易產生操作誤差。本法只須溶解及稀釋，直接由高壓液體層析可同時分離，定址二種主成分，而不受添加劑、色素、保存劑混存之影響。經廻歸方程式之統計分析及對同種試料之定量結果與B. P.方法之定量結果比較，證實其準確性及可行性。最適當之分析條件是以50% PEG 400溫液溶出主成分，經稀釋成約Trimethoprim 12 mcg/ml, Sulfamethoxazole 60mcg/ml. 使用μ Bon-dapak C 18為固定相以75% Methanol為移動相，254 nm之紫外光檢出器，流速2.0ml/min. 則Trimethoprim可以 0.02 AUFS, Sulfamethoxazole以 0.2 AUFS 分別定量。
