An FKBPI2/13 chimera with the 80s loop of FKBPI3 replacing the corresponding loop in FKBPI2 tightly binds the immunosuppressive agents FK506 and rapamycin and efficiently catalyzes peptidyl-prolyl cis-trans isomerization. However, the chimera's complex with FK506 does not inhibit calcineurin's phosphatase activity [Yang, Rosen & Schreiber (1993). J. Am. Chem. Soc. 115(2), 819-820]. The chimeric protein crystallizes in space group P1 and the crystals are always twinned. The twin composites are related by a twofold twinning axis parallel to the a axis. A resolution data set (1.5 A resolution) for a twinned crystal was collected at CHESS using 0.91 A X-rays and image plates. Preliminary molecular replacement using data between 15 and 3 A and the FKBPI2-FK506 crystal structure as the search model led to a clear solution with a residual of 34.2%. This 3 A resolution structure provides insight into the structural basis of twinning.