In situ nucleic acid hybridization was tested for the ability to detect bluetongue virus (BTV) nucleic acids in blood mononuclear cells. A standard protocol was devised and applied to the demonstration of BTV genetic sequences in cultured bovine mononuclear cells that had been infected in vitro. In situ hybridization using biotinylated single-stranded RNA probes, in the presence of 50% formamide at 50 C, demonstrated an intense, positive signal in 0.001-0.01% of the BTV-infected cultured mononuclear cells. The protocol was applied to isolated mononuclear cells from an experimentally infected heifer. No infected cells were observed by this method, although the blood specimens were obtained during peak viremia.