The complex N-AcPhe-tRNA(Phe).poly(U).80 S ribosome from human placenta was treated with puromycin taken in various concentrations. Based on the kinetic data of N-acetylphenylalanyl-puromycin formation, the association constant of puromycin with the acceptor site of the ribosome was estimated to be (3.96 +/- 0.84) x 10(4) M-1 at 37 degrees C.