X-ray quality crystals of a soluble murine class I H-2K b molecule complexed with three different peptide antigens were grown in several forms by streak seeding and macroseeding methods. Co-crystals with VSV-8 (RGYVYGQL), OVA-8 (SIINFEKL) and SEV-9 (FAPGNYPAL) peptides were grown either from NaH 2 PO 4 K 2 HPO 4 or from polyethylene glycol 4000 within the pH range 5· 0 to 7· 5, with the use of 4-methyl-2-pentane diol (MPD) as an additive. The VSV-8 crystals grew in space groups P1, with cell dimensions a= 63· 1 A ̊, b= 69· 1 A ̊, c= 72· 0 A ̊, α= 89· 9°, β= 77· 1°, γ= 123· 3° and P2 1 2 1 2, with a= 138.1 A ̊, b= 88· 6 A ̊, c= 45· 7 A ̊, and diffract to 2· 9 and 2· 3 Å, respectively. Crystals of the SEV-9 complex grew from similar crystallization conditions to those of the orthorhombic VSV-8 complex with similar cell parameters and diffract to at least 2· 5 Å resolution. Crystals of the OVA-8 complex were obtained from either phosphate (space group C2, a= 118· 7 A ̊, b= 61· 6 A ̊, c= 85· 3 A ̊, β= 108· 4°) or polyethylene glycol (space group P1, a= 64· 5 A ̊, b= 71· 0 A ̊, c= 66· 3 A ̊, α= 89· 7°, β= 95· 7°, γ= 123· 3°) and diffract to 3 Å resolution. The crystallization procedures used here significantly increased the rate and production of X-ray quality crystals.