Twelve different groups of fine mine dusts were examined for their in vivo aggressivity by intraperitoneal injection. Twelve months after injection of the dust, quantitative morphometric analysis of the lymph nodes was carried out to determine (a) the extent of the typical quartz reaction, (b) the in vivo cytotoxicity, (c) the size of the lymph nodes. Different fine dusts showed significant variations in parameter (a) and (c). In the second part of the investigation, the results of the lymph node tests and of in vitro cytotoxicity tests, and also the quartz contents of the dusts were fed into a data matrix and subjected to correlation analysis and principal component analysis. The following hypotheses were arrived at to explain the effect of the fine dust in vivo: (1) The amount of fine dust which penetrates into the lymph nodes is dependent on its in vitro cytotoxicity (Factor A). This is not related to the quartz content of the dust and is not inhibited by PVNO. (2) The specific fibrogenic activity of the fine dust within the lymph node is primarily dependent on its quartz content (Factor B). It appears that this effect is inhibited by PVNO. (3) The total fibrogenic activity depends on a combination of Factor A and B.