In a long-term intraperitoneal test with rats the quantitative and qualitative effect of unmodified natural silicious earth with and without surface coating was studied. The following results were obtained: Natural silicious earth (NSE) causes distinct silicotic reactions. Surface-coating of this dust does not reduce its harmfulness, but only an initial inhibition of the silicotic reaction can be observed. The effects of natural products (silicious earth, diatomaceous earth, diatomite) as well as of amorphous synthetic silica are compared and contrasted.