The compounds 3-ethoxy- ( 2), 3-butoxy- ( 3), 3-hexyloxy- ( 4), 3-octyloxy- ( 5), 3-decyloxy- ( 6) and 3-dodecyloxyjatrorrhizine chlorides ( 7) were synthesized and tested for their antimicrobial activity in vitro to evaluate structure-activity relationships. Substitution of the H with alkyl groups at C-3-OH led to significant changes in the antimicrobial activity. The antimicrobial activity of the substituted derivatives was 32 - 1000 times higher than that of jatorrhizine ( 1), which increased as the aliphatic chain was elongated and then decreased slightly when the alkyl chain exceeded eight carbon atoms. 3-Octyloxyjatrorrhizine ( 5) displayed the highest antimicrobial activity of all compounds. The LD (50) values of compounds 1 - 7 were more than 6000 mg/kg body weight, showing a low toxicity. The toxicities of compounds 2 - 7 were slightly lower than that of ( 1).