The effect of dead rock dusts from a Hungarian ore mine was studied in long term experiments. Histological changes caused by intratracheally introduced respirable mixed rock patterns (Porphyry, Enargite, Scarnic grained- and drill-cuttings) were examined and compared with standard DQ12 quartz samples 3, 6, 12 and 20 months after treatment. All three dusts proved to be fibrogenic, although in different extent. In the case of Porphyry rock patterns also storage type reaction developed. In the case of Scarnic dusts chronic pneumonitis, focal condensation of the reticular network and very mild fibrosis could be observed. The described changes have confirmed that there is a close connection between the destruction of macrophages and the fibrogenic effect. The free quartz content of the dusts, the amount and ratio of "protective" metal oxides (Al-, Ca-, K-, Mg-, Na-oxides) may be responsible for the differences in the histological reaction and the progression of fibrosis.