A total of 764 fresh clinical isolates were used to test a rapid method for determining lysine, arginine, and ornithine decarboxylase activity as well as arginine dihydrolase activity. The conventional Møller decarboxylase broth was tested in parallel with the rapid method on 234 Enterobacteriaceae and 140 non-fermentative Gram-negative rods. The 0.3% agar method was tested in parallel on 245 Enterobacteriaceae and 146 non-fermentors. All media were checked at half-hour or hourly intervals for up to eight hours, with the final reading taken after incubation for 24 hours at 37 degrees C. The rapid method detected 17 positive decarboxylase or dihydrolase reactions that were not detected by the Møller broth and 16 more than the agar medium when testing Enterobacteriaceae. The corresponding figures for the nonfermentative Gram-negative rods were three and two respectively. Lysine and ornithine decarboxylase were generally detected by the rapid broth in two to four hours' incubation while the arginine decarboxylase and dihydrolase were slower and required six to eight hours. This compares with overnight incubation as the general rule for the Møller broth and agar decarboxylases. The comparable accuracy of the rapid method with conventional techniques and the shorter incubation time required for detection of positive reactions make this procedure well suited to a routine clinical laboratory.