Three calves were sensitized with three doses of inactivated B T V l l UC8 strain and then experimentally infected with the homologous virus. In addition, four BTV-seronegative heifers were also experimentally infected with BTV-11. Granulocyte rich fractions of peripheral blood leucocyte CpBL-GRF) cultures from BTV-sensitized/infected calves and from control unexposed cattle were exposed in uitm with BTV-11. Histamine, leukotriene (Lp C, and prostaglandin (PG) D, were assayed in supernatant fluids. Plasma histamine levels increased in BTV-infected heifers from 10.1 f 2 ng/ml at Day 0 to 23.1 f 6.6ng/ml at Day 12 following virus exposure. In addition, in this experimental group the concentration of PGF,, (mean 551.97 f 243.54pg/ml) increased significantly (P < 0.05) compared with control cattle (mean 467.3 f 73.9 pg/ml). Bluetongue virus induced hlstamine and I,TC, release after in vitm infection of PBL-GRF. Release of LTC, was sigruficantly ( I ' < 0.05) higher in PBL-GRF cultures from sensitized and control animals than in unexposed PBL-GRF cultures. In contrast to these results, PGDz was not released after BTV infection of PBL-GRF in vitro. The histamine release caused by BTV was virus-specific and mainly mediated by an immunological reaction, since the rclease was significantly reduced by removal of cell surface immunoglobuhns.