Losses from pestivirus infection in a closed herd of cattle occurred over several years. In order to prevent further losses, controlled exposure of non-pregnant heifers to pestivirus from viraemic carrier animals was undertaken. Two initial experiments were conducted using either intra-nasal EDTA blood or field contact. Subsequently, other yearling heifers were inoculated with various dilutions of serum using subcutaneous, conjunctival and intra-nasal routes. Effective doses were determined. Neither inoculation nor contact infection produced any clinical illness. The highest dilutions of serum at which sero-conversion occurred were conjunctival, undiluted; intranasal, 10(-1) and subcutaneous 10(-5). With the subcutaneous route all heifers sero-converted at 10(-3). The results for the subcutaneous inoculations showed that the 50% infectious dose for cattle was not distinguishable from that determined in cell culture. Inoculation with a field strain of pestivirus in freeze-thawed serum has effectively and safely induced sero-conversion in heifers. Inoculation of all cattle at risk is considered necessary because no secondary transmission from inoculated heifers was observed.