A randomised, double-blind, placebo-controlled intervention study was carried out in order to investigate whether or not consumption of a dietary supplement containing probiotic bacteria plus vitamins and minerals over a period of at least 3 months in winter/spring affects the duration, frequency and severity of symptoms of naturally acquired common cold infections as well as cellular immune parameters. A total of 477 healthy men and women who had not been vaccinated against influenza were randomly assigned to a group who received daily the probiotic multivitamin and mineral supplement (verum) or a placebo for 3 (n=239) or 5.5 months (n=238). The cellular immune response was determined in 60 participants per study group by flow cytometry before and after 14 days of supplementation. The incidence of respiratory tract infections regarded as being virally induced was 13.6% lower in the verum group compared to the placebo group. During respiratory tract infection episodes the subjects recorded common cold and influenza-like symptoms daily. All symptoms were reduced in the verum group. A relative reduction of 19% in the total symptom score, 25% in influenza symptoms and 54% in the number of days with fever was found. The duration of these infections was not affected. Leucocytes, lymphocytes, in particular T-lymphocytes including CD4+and CD8+cells, as well as monocytes were significantly higher in the verum group during the first 14 days of supplementation compared to placebo.