The human Clara cell 16 kDa protein (CC10), isolated from lung lavage fluid, has been crystallized in two crystal forms. The first is in space group P1 and has cell parameters a = 43.04, b = 45.90, c = 51.29 A and alpha = 62.46, beta = 69.74, gamma = 69.43 degrees. Two molecules are present in the unit cell. The second form is in space group P222, with cell parameters a = 42.24, b = 84.06, c = 40.05 A and alpha = beta = gamma = 90 degrees, and four molecules per unit cell. Its diffraction pattern displays pseudo-body-centered symmetry. Both crystal forms diffract X-rays beyond 2.0 A.