The causative agent of enzootic bovine leukosis (EBL) is an oncogenic RNA virus named bonvine leukosis virus (BLV). At present, instead of bovine leukemia, the name bovine leukosis is preferentially used to avoid an erroneous association with leucaemia in man. In all European Community Countries the serological diagnosis of EBL has gradually replaced hematology. A number of these serological techniques are available to-date. In the Central Veterinary Institute in Rotterdam the agar gel immunodiffusion technique, the FAT and the ELISA are used as diagnostical tools. Based on own experiments it is provisionally concluded that BLV shedding via faeces and urine does not occur. Saliva has been found infective in three out of fourteen cases (21 per cent). There is no evidence of transmission of BLV in the sperm. Prostate fluid and sperm from seven experimentally infected bulls did not contain neither BLV antigen nor antibodies to BLV. Five calves born from five cows which had been naturally served by the mentioned bulls did not show sero conversion after an observation period of one year. The authors recommend to use in AI only sperm form bulls which are negative for antibodies to BVL. Preferentially other cattle at the same farm-enterprise should be serologically tested with negative results within three months before shipment of the sperm. The humoral and cellular immunological status of leucotic cattle are examined by the application of pig erythrocytes, tetanus toxoid, Bordetella- and Aujeszky vaccine. The seroloical reactions of leucotic cattle did not differ significantly from those of the "normal' controls. By contrast the cutaneous tuberculine reaction following administration of Mycobacterium microtii was significantly more obvious in the leucotic animals as compared to the controls.