From an economical standpoint, the colonization of the respiratory epithelium by Mycoplasma hyopneumoniae, the mollicute responsible for enzootic pneumonia in pigs which leads to a reduction in the weight gain/feed ratio, has serious consequences in very young animals; we show here that its is possible to efficiently protect these animals with the colostral antibodies raised in sows immunized during their gestation with M. hyopneumoniae BQ14 membranes. In an experiments using 9 sows and 75 piglets, the vaccinal preparation was found to protect 70 to 100% of the piglets (depending upon the presentation of the immunogen) as well as being totally innocuous. The best results were obtained with an adjuvant made of hydroxide aluminium gel. Of the piglets born to non-immunized sows, 8.3% exhibited typical pneumonic lesions. The vaccination procedure was found to enhance the elimination of M. hyopneumoniae from the respiratory tract of the piglets, as in 95% of the protected animals, the microorganisms could no longer be isolated. Conversely, M. hyopneumoniae was isolated from 89% of the piglets born to unvaccinated sows. A correlation was found between the protective effect and the rise in the titre of circulating antibodies directed against M. hyopneumoniae surface antigens. These results show that the plasma membrane of M. hyopneumoniae is a potential vaccine against porcine enzootic pneumonia. In particular, passively transmitted immunity via the colostrum provides the most precocious means of protection for the animals.