Ferricytochromes c were crystallized at low ionic strength by macroseeding techniques. Large crystals were grown by seed-induced self-nucleation which occurred anywhere in the drop, regardless of the location of the seed crystal. This unusual crystal-seeding method worked reproducibly in our hands, and X-ray quality crystals have been prepared of several ferricytochromes c: horse, rat (recombinant wild type), and two site-directed mutants of the latter, tyrosine 67 to phenylalanine (Y67F) and asparagine 52 to isoleucine (N52I). Crystals of any one of these four proteins could be used as seeds for the crystallization of any one of the others. All the crystals are of the same crystal form, with space group P2(1)2(1)2(1). There are two protein molecules per asymmetric unit. The crystals are stable in the X-ray beam and diffract to at least 2.0 A, resolution. Full crystallographic data sets have been collected from single crystals of all four proteins.