Human factor D, an essential enzyme of the alternative pathway of complement activation, has been crystallized. Crystals were grown by vapor diffusion using polyethylene glycol 6000 and NaCl as precipitants. The factor D crystals are triclinic and the space group is P1 with unit cell dimensions a = 40·8 A ̊, b = 64·7 A ̊, c = 40·3 A ̊, α = 101·0°, β = 109·7°, γ = 74·3°. The unit cell contains two molecules of factor D related by a non-crystallographic 2-fold axis. The crystals grow to dimensions of 0·8 mm × 0·5 mm × 0·2 mm within five days, are stable in the X-ray beam and diffract beyond 2·5 Å. © 1991.