Louis Willems' name is intimately linked with the history of prophylactic immunization in the nineteenth century. When he obtained his medical degreein 1849 contagious bovine pleuropneumonia or lung sickness was raging among the cattle population in most European countries. The disease is caused by Mycoplasma mycoides subspecies mycoides, but in the middle of the nineteenth century, during the battle between the miasmatists and the contagonistsmany doubted its contagiousness. From the start, Willems defended the contagiousness of the disease and noticed that animals that had survived an infection did not contract it a second time. He demonstrated that inoculation of the serous fluid from the lungs or from the pleural cavity of affected animals into healthy cattle led to pronounced local reactions. Later on, when these inoculated animals came into contact with diseased cattle they turned out to be immune. In his first trials he inoculated at the base of the tail or around the nostrils but this led to very severe reactions and frequently to death. He then started inoculating at the tip of the tail with muchbetter results. Most animals showed a more or less pronounced reaction at the inoculation site caused by the etiological agent itself. The tip of the tail was obviously a good choice; this was confirmed later by many authors and the procedure is still being used today in areas where the disease is still prevalent. Inoculation at other sites of the body, such as the neck or the dewlap, led to very severe reactions often followed bydeath. Willems also demonstrated that local inoculation at the tip of the tail not only immunized the animals against infection via the respiratory tract resulting from contact with diseased animals, but also against a second inoculation in the tail, in the neck or elsewhere. Material harvested from the inoculation site in the tail (so-called secondary "virus") could also be used as inoculum. Animals that showed no reaction to the first inoculation received a second inoculation after a few weeks. Not only in Willems' lifetime (by himself and by his contemporaries) was immunization as a result of inoculation repeatedly proved through experiment, but also later, in more recent trials. Failures were usually attributed to inoculation of already infected animals or to the use of badly stored or purulent inocula. Inoculation during the incubation period did not provide protection. The publication of his results created enormous interest in the matter in his country and abroad. in several countries commissions were founded, trials were initiated and several foreign observers came to visit Willems. In general his results were confirmed abroad, at least if the trials were conducted correctly; this was not always the case. A critical evaluation of Willems' work leads us to the conclusion that hisinitial concepts and results in the field of immunization were correct andlargely confirmed by the facts and the observations of others. As for one aspect Willems erred completely, that is when he claimed that the small corpuscles he had observed in the lesions were the causal agents of the disease. In most countries lung sickness was eradicated by sanitary measures and byslaughtering all infected cattle, but some countries such as Australia have combined Willems' inoculation procedure with other measures for a long time. Even today his method is still being used, for instance in Africa, but infectious lung material has now been replaced by inocula consisting of attenuated mycoplasma cultures.