1. Wild stable flies (Stomoxys calcitrans) feeding on heifers infected with bovine leukosis virus (BLV) carried viable bovine leucocytes in the midgut and proboscis that, when inoculated by the subcutaneous route into lambs aged 5 to 60 days, elicited the development of antibodies to glycoprotein (gp51) and polypeptide 25 (p25). 2. Antibodies were detected as early as one month later and persisted for an experimental period of 24 or 36 months. Uninoculated control lambs reared together with the experimental animals did not acquire the infection, indicating the lack of horizontal transmission. 3. S. calcitrans reared in the laboratory were intermittently allowed to feed on the skin of BLV-infected heifers and on five lambs over a period of 3-10 months. Although some of these lambs were bitten about 500 times, none developed antibodies to BLV (gp51 or p25) over observation periods of 30 or 36 months.