The in-vitro conditions under which coinhibition of viral interferon induction occurred when mammalian cell monolayers were exposed to bioactivated benzo(a)pyrene (50328) (BaP) in conjunction with representative particles of coal, asbestos (1332214), metals, and silicates were investigated. The Rhesus-monkey kidney cell line LLC- MK2 was used for induction and the Chang human conjunctival cell line clone 1-5c-4 was used for assay of interferon. Ao/PR/9/34 influenza and parainfluenza (Sendai) viruses were used for interferon induction and assay. BaP itself had no adverse effect on the induction process. However, when cell cultures were pretreated with BaP that was bioactivated by rat liver S9 homogenate, from 52% to 65% inhibition of interferon induction occurred. An 83% inhibition of induction occurred when BaP was incorporated with coal particles. When metals were individually mixed with bioactivated BaP, coinhibition of cellular interferon synthesis was also noted. Coinhibition of interferon induction by silicates and bioactivated BaP was not noted. The findings affirmed that enzymatically activated BaP in combination with different particles, the majority of which were occupationally related, acts to coinhibit a biologic defense mechanism, the induction phase of the interferon system.