Nine pregnant heifers were inoculated intravenously with infectious bovine rhinotracheitis virus (IBRV) in the sixth month of pregnancy. Tissues were collected from the fetus of a heifer killed 13 days postinoculation (PI), from fetuses of 6 heifers that aborted 16-27 days PI, and from mummified fetuses of 2 heifers that aborted 53 and 85 days PI, respectively. Control tissues were obtained from the fetus of a non-inoculated heifer that was killed in the seventh month of gestation. Tissues were fixed in 10% formalin, embedded in paraffin, and examined for viral antigen by immunohistochemistry, using biotinylated second antibody and alkaline phosphatase-labeled avidin-biotin complex. Antigen was detected in at least 1 tissue from the fetus of each inoculated heifer. Positive tissues included lung, liver, spleen, kidney, adrenal, and placenta. In several fetuses, antigen was identified in tissues from which virus was not isolated in cell culture. This appeared to occur when tissues had only a few small foci of infection or when tissues were severely autolyzed. The observation of viral antigen in tissues from mummified fetuses indicates that this technique may be useful in diagnostic laboratories to detect IBRV infection in tissues that are not suitable for virus isolation or for examination by the cryostat tissue section-fluorescent antibody technique.