The proportion of asialoGM1 positive cells and NK activity of murine splenic cells was reduced to almost zero one day after intravenous injection of rabbit anti-asialoGM1 antibody. The cells and the activity started to increase at the latest 3 days after the injection, although the proportion was far below that of the normal control. The proportion of asialoGM1 positive cells and NK activity increased more remarkably when 1,3-beta glucan, sizofiran, was administered intramuscularly one day after the antibody injection. The increases were dose related (50-1000 micrograms/mouse). The fact that sizofiran hastened the recoveries of splenic NK activity and asialoGM1 positive cells suggests sizofiran may have the activity to accelerate the differentiation of asialoGM1 positive NK cells.