The recombinant zymogen of the human complement protein factor D has been crystallized. Crystals were grown by vapor diffusion using polyethylene glycol 6000 as precipitant. Two crystal forms obtained at pH 5.4 belong to space group P21. The crystals grow to dimensions of 0.6 mm x 0.3 mm x 0.3 mm in three days, are stable in the X-ray beam, and diffract to 2.4 Å. © 1994 Academic Press Limited.