Mordenite, a fibrous-granular type of zeolites, was examined in CFY rats in long term in vivo experiments. After single intratracheal treatment, the lungs, cervical and hilar lymph nodes of the animals were processed at the end of the 1st, 3rd and 6th month and also the 1st year by routine histology, enzyme histochemistry and electron microscopy. After observing the effect of mordenite, dust-storing macrophage foci developed in the interstitium, showing minimal fibrotic tendency by the end of the 1st year. Electron microscopic examinations showed that the dust introduced was stored in the macrophages without structural changes. Energy dispersive X-ray microanalysis indicated that in intracellularly stored dust, the ratio of the two main elements: aluminium and silicon changed in favour of aluminium as compared to the original dust sample. This phenomenon was assessed as an increase in intracellular aluminium concentration, which inhibited fibrosis, exerting an antagonistic effect on silicon.