Analysis of drugs using HPLC (High Performance Liquid Chromatography) in pharmacokinetics studies and clinical tests is an effective means of elucidating the behavior of drugs and their metabolites in living organisms. Normally, when conducting HPLC analysis of drug-containing bio-samples, such as blood plasma and blood serum, etc., it is first necessary to eliminate proteins and other contaminants through pretreatment. This pretreatment generally consists of using an organic solvent, etc. to remove the proteins, and then performing centrifugal separation. However, not only are these pretreatment procedures cumbersome, analytical accuracy can be compromised due to the reliance on manually performed procedures. For this reason, it is becoming preferable to automate these types of procedures. Recently, advances have been made in the development of pretreatment columns designed specifically for removing proteins and other contaminants(1 to 4), and there is growing expectation that with these columns in combination with a column switching system, the pretreatment operations can be automated to provide better analytical accuracy and enhanced specimen processing capability. Here we report on the principles and features of a bio-sample analysis column switching HPLC system (Co-Sense for BA), equipped with the newly developed Shim-pack MAYI-ODS5 ) bio-sample pretreatment column and unique on-line dilution channel, in addition to application examples utilizing direct injection of blood plasma to perform drug analysis.