In order to investigate the binding of guinea pig lymphotoxin (GLT) to target cells, the reduction of survival ratio (S) after the pulsed exposure of target cells to GLT at 4 degrees C and subsequent incubation at 37 degrees C for 24 hr was examined in detail. The dose response of GLT, and the relation between the S value and the length of pulse treatment demonstrated the saturation and equilibrium of the GLT-binding. It was also shown that the GLT once bound to the target cells dissociated very slowly. Several sugars possessing a terminal beta-galactosyl residue had an inhibitory activity against the effect of pulse treatment. This suggests that GLT binds to receptors on the target cell surface, probably glycoproteins having beta-galactosyl residues as terminal sugar residues. The trypsin treatment of GLT-pulsed target cells restored survival ratios and this restoration was dependent upon the time of trypsin treatment. This suggests that the stage of the association of GLT to target cells changes from trypsin-sensitive into trypsin-insensitive.