Chemically defined diets require reconstitution and transfer to a delivery system. When reconstituted High Vivonex was noted in our Medical Center to be bacteriologically contaminated, we instituted a series of control procedures. We then reevaluated bacterial growth in reconstituted High Nitrogen Vivonex and diluted Isocal under ward conditions. The mixtures were prepared with sterile water versus tap water, using a hand washed blender versus a machine washed blender. We also investigated the bacteriological effect of blast freezing reconstituted High Nitrogen Vivonex. All preparations of the nonfrozen High Nitrogen Vivonex showed occasional low level contamination, although quantitative cultures did not show logarithmic growth over eight hours of observation. No growth occurred in the blast frozen High Nitrogen Vivonex or in the Isocal. We conclude that reconstituted High Nitrogen Vivonex and diluted Isocal may be prepared and hung safely for eight hours, and that blast freezing of High Nitrogen Vivonex is bacteriologically safe. As a result of our initial findings of bacteriologic contamination, we believe a program for bacterial monitoring of the tube feeding is desirable.