Abstract: The antibacterial activities of doxycycline, TMP alone and in combination were compared. 15 Clinical isolated and 15 laboratory strains were used in minimal inhibitory concn. (MIC) detn in vitro. The results suggested that the combined action of two drugs in the rations of 2:1, 1:1, 1:2 was not greater than the action of single effective drug when used alone in most cases, although synergistic activity against 3 strains, (S.peumococcus S. marcescens and S. hemolytic) and additive activity against 2 strains (E. coli and B. areogenes) were observed. The results of the protection test in vivo with S. marcescens, against which synergism was observed in vitro indicated that there was only additive activity in vivo when two drugs were taken simultaneously.