We report bacterial interference with urine osmolality measurements using an instrument based on the principle of freezing point depression. Although the exact nature of the interfering activity has not been defined, the phenomenon is associated with a bacterium, identified as Pseudomonas putida, and is removed from the specimens by filtration at 0.45 micron. The bacteria led to osmometer dysfunction presumably by acting as a nidus for crystallization and preventing proper supercooling of specimens.