We describe potential interference of procainamide with our high pressure liquid chromatography procedure for assaying theophylline (similar to that given by Orcutt et al.). In participating in the AACC-TDM quality-control program, we discovered procainamide interference on a column dedicated to theophylline and acetaminophen serum analyses. Following attempts at rejuvenation, this 7-month-old column was replaced with a new one, which gave base-line separation of theophylline and procainamide with no difficulty. We recommend close attention to the aging characteristics of columns and participation in a quality-control program.