RICHTER, E. A., B. KIENS, A. RABEN, N. TVEDE, and B. K. PEDERSEN. Immune parameters in male atheletes after a lacto-ovo vegetarian diet and a mixed Western diet. Med. Sci. Sports Exerc., Vol. 23, No. 5, pp. 517–521, 1991. The influence of a lacto-ovo vegetarian diet versus a meat-rich Western diet on in vitro measures of immune function was studied in eight male endurance athletes. Subjects consumed two different diets for 2 x 6 wk, separated by 4 wk on an ad libitum diet, in a cross-over design. Both diets consisted of 57 energy % (E%) carbohydrates, 14 E% protein and 29 E% fat. One diet was a mixed meat-rich diet (M) prepared with 69% animal protein sources, whereas the other diet (V) was a lacto-ovo vegetarian diet prepared with 82% vegetable protein sources. Blood for determination of leukocyte subpopulations and in vitro function was collected at the end of each diet period 36 h after the last training bout. Fiber content and P/S ratio of fatty acids were twice as high on the V diet as on the M diet. Training volume was similar on the two diets, and maximal aerobic capacity did not change during diet periods. The number of CD3+ (pan T-cells), CD8+ (mainly T suppressor cells), CD4+ (mainly T helper cells), CD16+ (natural killer cells), and CD14+ (monocytes) was similar after the two different diets. Similarly, proflierations of mononuclear cells after stimulation with interleukin-2 (IL-2), phytohemagglutinin, and purified derivative of tuberculin (PPD), as well as activity of natural killer cells in the unstimulated state and after stimulation with IL-2, indomethacin, and interferon-α (IFN-α), were identical after the two diet periods. It is concluded that, in male athletes, the concentration, comosition, and in vitro function of human blood mononuclear cells are unaffected by whether the diet in the preceding 6 wk has been a meat-rich mixed diet or a lacto-ovo vegetarian diet.