The aim was to investigate the influence of a daily mixed nutritional supplement during an 8-week arduous training programme on immune indices and mediators including circulating leucocyte counts; bacterially stimulated neutrophil degranulation; interleukin-6 (IL-6), cortisol and saliva secretory immunoglobulin-A (SIgA). Thirty men (mean (SD): age 25 (3) years; body mass, 80.9 (7.7) kg) received a habitual diet (CON, n = 15) or received a habitual diet plus an additional food supplement (SUP, n = 15). From weeks 0-6, CON received 14.0 MJ day(-1) and SUP received 19.7 MJ day(-1), and during a final 2-week field exercise in weeks 7 and 8, CON received 17.7 MJ day(-1) and SUP received 21.3 MJ day(-1). Blood and saliva were taken at rest after an overnight fast at weeks 0, 6 and 8. Body mass loss over the 8 weeks was greater in CON (CON, 5.0 (2.3); SUP, 1.6 (1.5) kg: P < 0.001). Training-induced decreases in circulating total leucocytes (CON: weeks 0, 8.0 (2.1); weeks 8, 6.5 (1.6) 10(9) l(-1), P < 0.01), lymphocytes (21%, P < 0.01) and monocytes (20%, P < 0.01) were prevented by the nutritional supplement. Saliva SIgA secretion rate increased approximately twofold by week 8 in SUP (P < 0.01) and was greater at week 8 compared with CON (P < 0.01). Circulating neutrophils, bacterially stimulated neutrophil degranulation, IL-6 and cortisol were similar in CON and SUP at week 8. In conclusion, a daily mixed nutritional supplement prevented the decrease in circulating total leucocytes, lymphocytes and monocytes and increased saliva SIgA output during an 8-week arduous training programme. The increase in saliva SIgA with nutritional supplementation during training may reduce susceptibility to upper respiratory infection.