AbstractThe senescent leaf disks fromBrassica rapaL. were floated on a medium containing14C‐L.‐leucine in the presence or absence of kinetin. The increase in radioactivity in protein fraction of kinetin‐treated disks was almost linear with time, while the increase in untreated disks incubated for 3 days started to slow down. The leaf disks were first incubated on the solution of14C‐L‐leucine and then transferred to either kinetin solution or water in order to study whether the slower incorporation is due to the decreased rate of protein synthesis or that of protein retention. The decrease in radioactivity of disks floated on kinetin was slower than that of disks floated on water. The slower decrease in radioactivity caused by kinetin was not due to an increased turnover rate, since the same phenomena were observed in the presence of cold leucine or casein hydrolysate solution. These results suggest that kinetin retards the decomposition of protein in leaf disks and does not stimulate the synthesis of the bulk o