TRYPSIN is used extensively for dissociation of tissues and in the preparation of suspensions of living cells1. The cell-dissociating effect of this enzyme has been attributed to tryptic degradation of cell binding materials1; however, theoretically, other possibilities exist and the question whether the characteristic enzymatic properties of trypsin are actually involved in cell separation seems not to have been adequately tested, though such information would be of some general interest. Furthermore, since trypsin attacks selectively peptide linkages adjacent to lysyl and arginyl residues, clarification of the relationship between its cell-dissociating activity and its lytic properties would be useful in considerations of the mechanism of cell binding. Trypsin inhibitors offer a direct approach to this matter. It was reported that trypsin inactivated by diisopropylflurophosphate (DFP) did not dissociate adult rat liver cells2. In the work recorded here, the effects of soybean and ovomucoid trypsin inhibitors on the dissociation by trypsin of embryonic tissue were examined. The interaction between these inhibitors and trypsin is considered to be specific and stoichiometric3–5 at concentrations equimolar to and higher than that of the enzyme, the inhibitors rapidly and completely abolish its tryptic activity. According to available results3,5, the molecular weights of trypsin and trypsin inhibitors used in this investigation were of the following order of magnitude: trypsin—24,000; soybean trypsin inhibitor—24,000; ovomucoid trypsin inhibitor—28,000.