The complex nature of "all in one" formulas for TPN, makes it difficult to ensure the stability of the lipid emulsions in these preparations. Despite the fact that studies on this issue are increasing, official guidelines on maximum acceptable particle size and the methods to control it are still lacking. The objective of this study was to assess the stability of a lipid emulsion in four standard mixtures also containing amino acids, glucose or F-G-X, electrolytes and vitamins or trace elements. Lipid stability was assessed visually through electronic counts and microphotographs. The pH of mixtures ranges between 5.3 and 5.9, not changing with time nor storage temperature. Visual observation showed a possible rupture of the emulsion in one mixture after storing 96 hours at room temperature, but not when stored at 4 degrees C. Mean diameter of fat particles did not change. The percentage of droplets having a diameters under 1.2 micron was higher than 90%, although particles greater than 6 micron appeared in all mixtures. Micrographic observation revealed the presence of aggregates with a diameter greater than 8 microns in one mixture. According to the results obtained, one of the mixtures may be considered physically unstable. The clinical viability of this mixture and of the other three stable mixtures, depends on the limits considered acceptable regarding particle size. Due to the slight composition differences of the mixtures studied, it is difficult to explain the difference in behavior observed. Therefore, it would be adviceable to study standard "all in one" mixtures of known stability, while waiting for more conclusive studies to be performed, especially in the case of critical mixtures.