Two studies have been carried out to evaluate the prophylactic and therapeutical effect of a 30%-extract from the coffee-bean seeds Coffeae arabicae on infectious diseases in newborn calves. 1. Within a large cattle-herd, which endemically showed a high proportion of infections within the gastroenteric and/or respiratory systems in calves, a randomised placebo-controlled double-blind study has been done. 50 newborn calves were given a subcutaneous injection of 10 ml Coffea-preparation 30% on first and third day of life. Another 50 calves received physiological saline as control. An index was set up which allowed to daily evaluate and compare body-temperature, consistency of feces, exsiccation-degree and breathing-rate of the animals. Besides this the number of therapeutical interventions and the number of days with disease-symptoms were recorded. Calves treated with Coffea-extract showed: on first and second day of life less animals with body-temperature below physiological values (p < 0.001 or 0.1 resp.), during the first period of diarrhea (between fourth and sixth day) significantly lower tendency of diarrhea (p < 0.1; 0.001; 0.005 resp.), after the second period of diarrhea (around the 9th day of life) a better and quicker recovery and a lower tendency of exsiccation (p < 0.05 on day 10 and 11) as the control-calves. Besides this the average duration of illness was shorter (4.7 instead of 7 days) and the average number of therapeutical interventions were less (3.1 instead of 4.5) than in control-calves. 2. Within four cattle-herds endemically showing a high rate of diarrhea in newborn calves the morbidity in a total of 371 animals could be dropped from about 45% to 10% by prophylactic administration of one to three s.-c.-injections of 10 ml Coffea-preparation together with one or two million i.U.Vit.A. one time perorally. For prophylactic use two injections of coffea preparation. on day 1 and 4 of life proved to be efficient under the given circumstances. Therapeutically the daily administration of a combination of Coffea-extract together with oral drugs containing tannic substances and diet-feed could reduce the mortality in animals with acute disease to about 30%.