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, proliferations 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-alpha (IFN-alpha), were identical after the two diet periods.(ABSTRACT TRUNCATED AT 250 WORDS)