We have prepared large quantities of highly purified tetanus toxin by both the culture filtrate and cell extract procedure. We have separated the toxin molecule into its heavy chain (100,000 daltons) and its light chain (50,000 daltons) components by two procedures. One is by the modification (van Heyningen, 1976) of the ultragel filtration procedure of Matsuda and Yoneda (1974). The other and more promising procedure is by preparative polyacrylamide gel electrophoresis. There is proteolytic activity in the purified toxin which is inhibited by phenylmethylsulfonyl fluoride (PMSF). After separation into the heavy (H) and light (L) chain components, the proteolytic activity appears to follow the H chain. The L chain appears to be more stable and soluble than the H chain. We thus chose the L chain to start our sequence efforts and have determined some 22 residues from the amino terminal. We have also treated the L chain with cyanogen bromide and have fragmented it into at least ten peptides which are separated by SDS-urea polyacrylamide gels. We are continuing these efforts and should make considerable progress towards determining the complete amino acid sequence of L chain during the coming year.