Nuclear-resonance spectra of phosphatidylcholine (PC) vesicles are broadened by spin-labeled PC. The broadening of the N-methyl proton line is proportional to spin-labeled PC concentration; the broadening by 1 molecule in 100 of spin-labeled PC is 9.7 Hz at 35 degrees C; the broadening is not due to collisions between vesicles, exchange of spin label between vesicles, or fusion of vesicles. These findings are proof of rapid diffusion in the plane of the PC bilayer (lateral diffusion). A further study of the broadening reveals the molecular frequency of the translation step for lateral diffusion to be much greater than 3 x 10(3) sec(-1) at 0 degrees C.