HA3, a 70 kDa haemagglutinating protein, is a precursor form of HA3a and HA3b, the subcomponents of Clostridium botulinum type C 16S progenitor toxin. In this report, recombinant HA3 protein was overexpressed in Escherichia coli, purified and crystallized. Diffraction data were collected to 2.6 A resolution and the crystal belonged to the hexagonal space group P6(3). Matthews coefficient and self-rotation function calculations indicate that there is probably one molecule of HA3 in the asymmetric unit. A search for heavy-atom derivatives has been undertaken.