Reaction centers (RCs), integral membrane proteins that mediate the conversion of light into chemical energy, were crystallized by two different vapor diffusion techniques. In one method, small amphipathic molecules (1,2,3-heptanetriol and triethylammonium phosphate) were added to the RCs that had been solubilized in detergent. In the second method, crystallization occurred near the phase boundaries of a two-phase system created by the addition of polyethylene glycol and NaCl to RCs in octyl beta-D-glucoside. Several different crystal forms were obtained; two were analyzed by x-ray diffraction. One was monoclinic (space group P2) with beta = 105 degrees, and a = 70 A, b = 105 A, and c = 85 A, two RCs per unit cell, and one RC per asymmetric unit; the crystal diffracted to 3.5 A at 17 degrees C. The other crystal form was orthorhombic (space group C222) with a = 185 A, b = 170 A, and c = 105 A, with eight RCs per unit cell and one RC per asymmetric unit. Reversible light-induced EPR signals of the primary donor (bacteriochlorophyll dimer) showed that the RCs in the crystal were fully active. From the angular dependence of the EPR signal the molecular g anisotropy of the bacteriochlorophyll dimer was deduced to be g perpendicular - g parallel = (64 +/- 3) X 10(-5). Linear dichroism measurements were performed on the monoclinic crystal. The two bands at 535 and 544 nm assigned to the Qx transitions of the bacteriopheophytins were resolved and preliminary orientations of some of the pigments were obtained.