A series of 53 isoquinoline alkaloids and their N-oxides have been tested for their cytotoxicity against A-549, HCT-8, KB, P-388, and L-1210 cells. These alkaloids include two tetrahydroprotoberberines, two protoberberines, six aporphines, one morphinandienone, five oxoaporphines, seven phenanthrenes, one spirobenzylisoquinoline N-oxide, nine aporphine N-oxides, seven benzyltetrahydroisoquinoline N-oxides, one benzylisoquinoline N-oxide, one protopine N-oxide, three tetrahydroprotoberberine N-oxides, four pavine N-oxides, and four phenanthrene N-oxides. The results are discussed on the basis of structure-activity relationships.