The crystal structure of recombinant rat prostatic acid phosphatase in complex with the inhibitor L(+)-tartrate was determined to 3-A resolution with protein crystallographic methods. The inhibitor binds at the carboxyl end of the parallel strands of the alpha/beta domain. One of the carboxyl groups of the tartrate molecule interacts with the conserved residues Arg-11, His-12, and Arg-15, which form part of the phosphate binding site. Furthermore, the C2 and C3 hydroxyl groups interact with His-257 and Arg-79. The second carboxyl group is close to Arg-79 but makes no direct hydrogen bonds to the protein. A sequence comparison between tartrate-sensitive and -resistant acid phosphatases suggests that these enzymes have different three-dimensional structures.