Growth at 42 degrees C is advocated to differentiate species of the fluorescent pseudomonas group as well as to differentiate other nonfermentative bacteria. Methodologies vary in the performance of the test, resulting in differing and often discrepant results between investigators. During this evaluation, the test was performed by inoculating 3 ml of Trypticase soy broth with a loopful of an overnight broth culture. Growth in the 42 degrees C tube was judged as heavy or slight after 24 and 48 h incubation at 41.5 +/- 0.5 degrees C. Pseudomonas aeruginosa grew abundantly after overnight incubation, whereas 16 of 74 isolates of P. putida (22%) showed slight turbidity in the broth after 24 or 48 h which could not be regarded as an inoculum effect. Trypticase soy agar was used in conjunction with Trypticase soy broth, with the growth again judged as heavy or slight. Growth of P. putida on slants was still seen in some cases (6%) although the number of strains showing growth had declined. Triphenyltetrazolium chloride was added to Trypticase soy broth (0.005%) as a color indicator of growth. Strains of P. putida, although showing visible evidence of growth, gave no color change when compared with the 35 degrees C control. The constancy in test results using nonfermentative bacteria is not only method dependent but also strain dependent. Although the test for growth at 42 degrees C is important as a taxonomic tool when used under controlled conditions, other tests such as acetamide are preferred as a substitute for use in the clinical laboratory.