A direct injection method for the determination of nonpeptidic inhibitors of human leukocyte elastase in plasma, using column switching techniques, has been developed. In the technique, a heart-cut fraction is collected from a size exclusion column to separate the drug and metabolites from the plasma protein. Peak compression, concentration and additional purification are performed by a collection column. The components are flushed forward onto an analytical column for the separation of the drug component from the metabolites. The parallel processing of the technique gives it a number of advantages over conventional extraction procedures and many column switching procedures in terms of speed and effort required in the analysis of samples. The technique's advantages and the precision and accuracy of the assay are discussed.