The immunogenicity, innocuity and possible teratogenic effects of the monovalent modified-live vaccine against bluetongue (BT) virus (BTV) serotype 2, manufactured by Onderstepoort Biological Products in South Africa, was evaluated in cows. Twenty-one cows, 14 of which were at different stages of gestation, were vaccinated with 2 ml of monovalent vaccine; two served as unvaccinated controls. After immunisation, 16 vaccinated and the 2 unvaccinated controls were kept in the field; the remaining 5 pregnant cows were maintained in an insect-proof stable with a controlled environment. Blood samples were taken from field cattle once a week for two months and from the stable cattle three times a week. All samples were screened for the presence of BTV and for BT antibody using the competitive enzyme-linked immunosorbent assay (c-ELISA) and the virus neutralisation (VN) test. Intravenous egg inoculation, followed by two blind passages in Vero cells, was used to isolate BTV-2 from ethylene-diaminetetra-acetic acid (EDTA) blood samples and virus titres in viraemic animals were determined. After immunisation, 9 of the cows developed a viraemia which commenced on day 7 post vaccination (pv) and lasted for three weeks. The virus titres were never higher than 10(2.8)TCID50/ml with the highest titre observed on day 14 pv. None of the vaccinated animals developed clinical symptoms that could be attributed to BTV; after three weeks all animals showed a serological response to BTV-2. In the c-ELISA, antibodies were detected from day 7 pv while in the VN test, antibodies were observed from day 21 pv. All pregnant cows completed their gestation: 13 gave birth to healthy calves, while one of those in the field group, vaccinated at the six months gestation, delivered a calf with prosencephalic hypoplasia, possibly developed during foetal organogenesis prior to vaccination. Fourteen months after immunisation the stabled cows were challenged subcutaneously by administering 2x10(6.8)TCID50 BTV-2 Italian isolate. A third group of 4 cows was also inoculated with the BTV-2 Italian field isolate, as described for the second group and was used as the unvaccinated positive control group. Vaccinated cows had a detectable viraemia only on day 14 pv and virus titres were very low. Virus titres never exceeded 10(2.3)TCID50/ml, while the unvaccinated group developed a long and intense viraemia, peaking on day 14 pv with a titre of 1.18x10(4). It is concluded that the BTV-2 modified-live vaccine used in this study was a harmless and effective immunogen that did not cross the placental membrane.