Thermal mud is a therapeutic agent whose antirheumatic effectiveness is optimized by a process of maturation. The maturation of thermal mud was followed at 48 degrees C under controlled conditions by measuring physical and biochemical changes due to the growth of colonizing thermophilic microorganisms. Thermogravimetric measurements allowed us to identify the building up of an organic component including phospholipids and in particular a previously recognized sulfoglycolipid, which was further purified. The compound may be responsible for the antirheumatic effect of the mud and is produced by the colonizing species which develop in a period of maturation subsequent to that of production of photosynthetic pigments.