The human lymphocyte receptor CD5, a key regulator of immune responses, is involved in the modulation of antigen specific receptor-mediated T cell activation and differentiation signals. CD5 is a membrane glycoprotein which belongs to the group B scavenger receptor cysteine-rich (SRCR) superfamily for which no structural information is available. The most conserved membrane-proximal SRCR domain of CD5 (domain III) has been expressed in HEK-EBNA-293 cells. Although the yield of the purified protein was at the level of micrograms, well diffracting crystals have been obtained. The crystals belong to a tetragonal space group P4(1)22 or P4(3)22. They contain two molecules per asymmetric unit and diffracted to 2.5A resolution using synchrotron radiation. The strategy shown here to produce, isolate and crystallize CD5 domain III can be used for other mammalian proteins difficult to produce for structural or other biophysical studies.