The plasma membrane frac-tion isolated from primary leaves of 14 d soybean shoot showed protein kinase (PK) and protein phosphatase (PP) ac- tivities (Fig. 1), and these activities declined dramatically when the leaves underwent senescence-inducing treat- ment (Table 1). 6-BA pretreatment, which showed retarding effect on soy- bean leaf senescence, could sustain the activities of both Pigs and PPs during the senescence-inducing treatment (Table 1). The results of autoradiogra- phy suggested that the main PKs whose activities declined during leaf senes- cence were calcium-dependent protein kinases (Fig. 3). The potent and specif- ic inhibitor of protein phosphatase 1 and 2A, okadaic acid, could fasten leaf senescence. These results mplied that retaining the balance of plasma mem- rane protein phosphorylation/dephos- phorylation was the key action of exoge- nous 6-BA in retarding leaf senescence.