The anti-Lewis a mouse immunoglobulin CF4C4 (IgGl, κ) Fab has been crystallized from 58% saturated ammonium sulfate in space group P1; unit cell dimensions a = 43.4 A b = 41.7 A , c = 62.0 A , α = 72.7 ^o, β = 96.6 ^o, γ = 100.1 ^o. X-ray diffraction data have been measured beyond 3.0 A Bragg spacing. The crystal structure has been determined by molecular replacement methods, using as search models the constant and variable domains of the mouse immunoglobulin McPC603 (IgA, κ) Fab. The crystallographic residual for the data 5.0 to 4.0 A, is 0.47. The approximate 2-fold axis relating the VL and the VH domains forms an angle of 164 ^o with the 2-fold axis relating the constant domains. The crystal packing is reasonable.