To investigate a possible biological activity of iridoid glucosides, six compounds, aucubin, catalpol, gardenoside, geniposide, rehmannioside and swertiamarin, were studied in relation with their potential influences in RNA and protein biosyntheses in murine tumor cell, sarcoma 180, in vitro. Protein biosynthesis was slightly inhibited by aucubin, gardenoside and swertiamarin. Degree of inhibition of RNA biosynthesis by those iridoid appeared to be more sensitive than that of protein biosynthesis. When aucubin was pretreated with ${eta}-glucosidase$ to produce its genin form and the sarcoma 180 cells were exposed to this aucubigenin, the protein and RNA biosyntheses in the cells were profoundly inhibited. The results indicate that a biologically active from of iridoid compounds is the hydrolytic products of glycoside, i.e. genin form. It is also suggested that sarcoma 180 cells used in the experiments appear to lack of ${eta}-glucosidase$, since the inhibitory actions of iridoid glucosides were so slight that those glucosides were not hydrolysed by the enzyme to their genin forms.