A discontinuous preparative polyacrylamide gel electrophoresis system has been developed and used to purify both the nicked and unnicked forms of tetanus toxin. The system was also used to prepare purified H and L chain peptides from the nicked toxin. The results show that the endogenous protease(s), which convert unnicked toxin to the nicked form, produce multiple species of nicked toxin, and heterogeneity in the H and L chains. The major amino termini of the toxins and their peptide components are: extract toxin, proline; filtrate toxin, proline, serine and asparagine; L chain, proline; and H chain, serine and asparagine. The L chain is located in the amino terminal position of the toxin molecule and the H chain the carboxy terminal end. A model is proposed to explain these results. Using the analytical ultracentrifuge, we have determined the molecular weights of extract and filtrate toxins to be 140000 +/- 5000 and 128000 +/- 3000, respectively. Using SDS-polyacrylamide gel electrophoresis we estimate the molecular weights of the H and L chains to be 87000 and 48000 daltons, respectively. Circular dichroic spectra of the toxins and their peptide components indicate that: the major tryptophanyl band in the toxin is contributed almost entirely by the H chain, the microenvironments of all the aromatics and disulfides in the two toxins appear to have small if any differences, the two toxins show little difference in their ordered secondary structure, and the two peptides when separated from one another still retain 80% of the helical structure that is present in the intact toxin but show a considerable loss of beta-structure. The crystalline form of the nicked toxin has a hexagonal symmetry with two dimensional reciprocal lattice constants of 1/150 A-1 and 1/150 A-1. The crystals appear to belong to the two dimensional plane group P6 suggesting that each unit cell contains 6 or a multiple of 6 toxin molecules.