Abstract: A new attenuated strain (CNCM Institut Pasteur I-1642) of the virus causing the swine disease known as porcine reproductive and respiratory syndrome (PRRS) is described, along with an attenuation and replication procedure of the same by using a new clone obtained from MA-104 monkey kidney cells (CNCM Institut Pasteur I-1643). Because of the innocuousness on swine and the high immunogenic activity, the said new attenuated strain allows the obtaining of vaccines and diagnostic kits which make possible both the early diagnosis of PRRS and an efficient preventive treatment of such disease.