Five Clostridium botulinum strains were used in the present work, two of type C, C-Stockholm (C-ST) and C-CB19, and three of type D, D-South African (D-SA), D-1873, and D-CB16. The toxins, except for those of C-CB19 and D-CB16, were purified, and antisera were prepared in rabbits. To clarify the antigenicity of the toxins, neutralization and agar gel double-diffusion tests were performed. Anti-C-ST toxin serum neutralized two kinds of type C 1 toxin to a similar extent. Antisera against D-SA and D-1873 toxins, however, showed different neutralizing activity toward three type D toxins. Precipitin lines formed between D-SA and D-1873 toxins, and their antisera spurred to each other. From anti-D-SA toxin serum, two neutralizing fractions, one which neutralized D-SA, D-1873, and D-CB16 and one which neutralized only D-SA, were obtained. These results indicate that the antigenicities of D-SA and D-1873 toxins are not identical. Anti-C-ST toxin serum produced cross-neutralization on type D toxins, although the neutralization titer differed depending on the kind of toxin used. A precipitin line was formed with D-SA toxin, but not with D-1873; the developed line spurred to the C-ST toxin precipitin line. Two anti-D toxin sera also caused cross-neutralization on type C 1 toxins. However, the neutralizing activity of each serum to the same type C 1 toxin was different, and only anti-D-SA toxin serum developed a precipitin line with C-ST toxin which spurred to the D-SA toxin precipitin line. From anti-D-SA toxin serum, two different fractions capable of neutralizing C 1 and D toxins were obtained; one neutralized C-ST, C-CB19, and D-SA toxins, but not D-1873 and D-CB16, and the other neutralized all five toxins. There may be at least two common parts among C-ST, C-CB16, and D-SA toxin molecules.