A procedure for preparation of electrophoretically and serologically homogeneous neurotoxin and a highly purified hemagglutinin from the culture fluid of Cl. botulinum A, strain 501 is described. The yield of neurotoxin with specific activity of 80-100 X 10(6) DLM/mg of protein is 5-20%. Neurotoxin has a molecular weight of 150,000, sedimentation coefficient of 7.1S, pI of 6.2-6.3; the maximum of its fluorescence corresponds to 332 nm. The toxin molecule contains 4 SH-groups. Neurotoxin consists of two subunits with molecular weights of 98,000 and 56,000. The storage of neurotoxin at -20 degrees C causes inactivation and electrophoretical heterogeneity of the protein. The inactivation leads to an alteration of the toxin molecule charge, a shift of the lambda max of fluorescence and a loss of the SH-group reactivity without affecting the molecular weight or serological properties of the protein. The data obtained suggest that the conformational state of toxin molecules is essential for its biological activity.