The in-vitro influence of different ranks of coal dust on viral induction of interferon, multiplication of influenza virus, and interferon mediated antiviral cellular resistance was investigated. The Ao/PR/8/34 influenza and sendai virus strains were used. Rhesus- monkey kidney and human Chang conjunctival cell lines were used for induction and assay of interferon, respectively. Coal dust ranks used were anthracite (8029105), bituminous, subbituminous, lignite (8002537), and peat. From 0.5 to 2 milligrams per milliliter (mg/ml) of coal dust were added to monkey kidney cell cultures, and the cultures were incubated for 24 hours at 37 degrees-C. Residual medium was decanted, inactivated influenza virus was added, and cultures were incubated for another 2 hours. Following centrifugation, cell cultures were passed through filters and assayed for interferon activity by an immunofluorescent cell counting technique. Cell survival in the presence of 0.5, 1.0, and 2.0mg/ml quantities of coal dust was 40, 50, and 100 percent, respectively, regardless of coal rank. Coal of high rank such as anthracite and bituminous depressed interferon activity by about 66 percent. The lower coal ranks had less adverse effects. The inhibitory activity of coal dust was rank related. The ability of interferon to confer anti viral cellular protection was not affected by the presence of any coal rank. An inverse relationship was observed between virus growth and interferon concentrations in the presence of coal of different ranks. These observations suggest that the differences in viral growth reflect the ability of different coal ranks to affect interferon production. The author concludes that the inhibitory effect of coal dust on viral interferon induction is related to coal rank.