To examine how autolysis, fixation duration (6–72 h) and formalin concentration (2.5–25%) influence histology and IHC of canine distemper virus (CDV, Morbillivirus canis), interferon-β (IFN-β), and selected IFN-stimulated genes (ISGs), the study was conducted using an in vitro model based on persistently CDV-infected and non-infected DH82 cells (canine histiocytic sarcoma cell line). This evidence concerns the gene IFNB1 and histiocytic sarcoma.