The activities of the free base and ribonucleoside forms of cytokinins bearing saturated and unsaturatedN6-isoprenoid side chains have been examined in callus cultures derived fromPhaseolus vulgariscv. Great Northern,P. lunatuscv. Kingston, and the interspecific hybrid Great Northern×Kingston. In callus of cv. Great Northern, cytokinins bearing saturated side chains (N6-isopentyladenine,N6-isopentyladenosine, dihydrozeatin, and ribosyldihydrozeatin) were always more active than the corresponding unsaturated analogs (N6-[Δ2-isopentenyl]adenine,N6-[Δ2-isopentenyl]adenosine, zeatin, and ribosylzeatin). In callus of cv. Kinston, the cytokinins bearing unsaturated side chains were either more active or equally as active as the saturated compounds. These differences in cytokinin structure-activity relationships were correlated with differences in the metabolism of14C-N6-(Δ2-isopentenyl)adenosine. In Great Northern tissues, this cytokinin was rapidly degraded to adenosine; in Kingston tissues, the major metabolite was the corresponding nucleotide. The growth responses of callus of the interspecific hybrid were intermediate between the parental tissues, and the metabolism of14C-N6-(Δ2-isopentenyl)adenosine by the hybrid callus exhibited characteristics of both parental tissues. The results are consistent with the hypothesis that the weak activity of cytokinins with unsaturated side chains in promoting the growth of Great Northern callus is due to the rapid conversion of these cytokinins to inactive metabol