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Among the list of 37 Citation Classics in Clinical Chemistry published in the March issue of this journal (1) were 7 dealing with analytes of intermediary metabolism (2-8). Two each covered measurement of urea, glucose, and calcium, whereas one described the combined assay of lactate and pyruvate. All were published in the period 1962-1968, when modifications to methods for these commonly measured substances were welcome even as automation slowly took over much of their routine testing. Chaney and Marbach (2), in their paper from the Chaney Chemical Laboratory in Glendale, CA, "Modified reagents for determination of urea and ammonia" (Fig. 1), simplified the catalyzed indophenol reaction for the determination of ammonia by combining four reagents into two, each stable for 60 days or more; urea was determined after conversion to ammonia and carbon dioxide by incubation with urease for 20 min at room temperature or 5 min at 60 [degrees] (now there's a stable enzyme!). Although this is a labor-intensive manual procedure, most readers will probably not recall the diacetyl monoxime method it largely replaced, which required heating a serum filtrate in strong sulfuric acid in a boiling water bath for 20 min. The 1554 citations received by this paper through 1995 were well deserved.