The use of osmolality as an aid to establishing consistent fixation quality: studies on the kidney of the domestic fowl.

1. Using the proximal convoluted tubule of the fowl kidney as indicator tissue the immersion application of fixatives of differing composition and osmolality was studied. 2. The best results were obtained with Dalton's buffered osmium tetroxide and with sodium cacodylate buffered glutaraldehyde followed by osmium post-fixation. 3. The most satisfactory component concentrations in both cases were those which most reproduced a total osmolality close to that of fowl plasma. With both types of fixation the ultrastructural image of the kidney tubule was sensitive to changes in fixative osmolality and differences were clearly identified over a 30 to 50 mOsm range. 4. With glutaraldehyde/osmium double fixation it was found that it was the total osmolality of the glutaraldehyde buffer combination that was to be considered in a study of osmolotic effects, not the osolality of the buffer vehicle alone. 5. The 30 to 50 mOsm difference in fixative osmolality that was able to be detected by studying the image of the kidney tubule was of the order of magnitude commonly experienced as experimental error in the laboratory as a result of changes in reagent batches or error in making up the solutions. The measurement of osmolality therefore provides an ideal quality control to the laboratory make-up of fixative solutions.