The effect of a continuous infusion of a soybean oil emulsion on immune function was evaluated in 40 malnourished patients who were randomized to receive preoperatively either a 25% glucose-5% amino acid solution (group G) or a 15% glucose-3.3% Intralipid-5% amino acid solution (group G-F). Average length of total parenteral nutrition (TPN) was 10.3 +/- 0.9 days for group G and 9.0 +/- 0.8 days for group G-F. Initial nutritional status and response to TPN were similar for both groups. Immune function was assessed before TPN and after nutritional repletion prior to surgery for each patient. The levels of immunoglobulins, C3, C4, circulating B lymphocytes and T lymphocytes, suppressor T lymphocytes, natural killer cell activity, and monocytes were normal before TPN and after nutritional therapy. However, the total number of T cells and helper T cells were low before TPN and remained so after TPN. In addition, lymphocyte function measured by the lymphocyte blastogenic response to phytohemagglutinin and pokeweed mitogen was depressed prior to TPN and was not improved by either regimen. Neutrophil chemotaxis and bactericidal activity were not affected by either nutritional regimen while neutrophil phagocytosis was enhanced before TPN and remained elevated throughout TPN with either regimen. There were no differences in infection rates during TPN. The addition of Intralipid to the TPN regimen did not alter immune function in these patients who showed depressed cell-mediated immunity before TPN compared with the standard glucose TPN regimen.