We investigated the effect of Thymostimulin (TP-1) (a bovine thymic extract) on some clinical and immunological parameters in hospitalized aged humans. Forty aged patients were randomized into two groups, one received no therapy and the other was treated with TP-1 for 3 mth. The Ig and complement levels, sheep rosette-forming cells, absolute lymphocyte numbers and sedimentation rate were measured on days 0, 90 and 180. Moreover, the incidence of infections was determined. The baseline immunological data of the patients studied were within the normal range. These data remained unchanged after TP-1 treatment. In the TP-1 treated group we observed a significant reduction of infections (P=0.023) in comparison to the control group. A significant reduction of the sedimentation rate was also observed in the treated group.