LYOPHILIZATION has been used to preserve viable microorganisms for extended periods of time1. Recently, it has been reported that ribosomes functional in in vitro protein synthesis can be isolated from lyophilized fungi2 and that lyophilized Escherichia coliribosomes retain fully their capacity for poly U-directed phenylalanine incorporation after 5 months storage at room temperature over P2O5 (ref. 3). We have now compared the sedimentation profiles and poly U-directed phenylalanine incorporating activity of three types of rabbit reticulocyte ribosome preparations immediately after isolation and after freezing or lyophilization and storage for various times.