Wounds caused in the dorsal musculature of guinea pigs were sutured with various types of threads and the histological reactions caused by the latter were studied. Light and electron microscopic methods revealed that the best resorption can be achieved with Dexon threads. This synthetic thread is resorbed in 14 to 16 weeks causing only a minimal histological reaction and without cicatrization in its vicinity. Around threads which are not absorbed a connective tissue capsule is formed; this one is thin in the case of Ethilon and Chrom cat gut and somewhat more marked when Rixo or Supramid threads are used. The histological reaction is the most intense around the traditional linen thread which leads to cicatrization. In the resorption of Dexon a decisive role is attributed to the giant cells of the foreign body while in the case of non-absorbable synthetics and linen threads the fibre forming roles of the histiocytes, mastocytes and fibroblasts dominate.