The Gram-negative bacteria can cause very serious diseases. One of their pathogenetic factors is the endotoxic lipopolysaccharide (LPS). This molecule is a component of outer membrane of Gram-negative bacteria. The LPS is released from the living bacteria in a modest measure, but after killing off bacteria by antibiotics a larger quantity of endotoxin is released into the surrounding, This molecule can induce different biological effects including pyogenicity and toxic shock. The LPS is an amphiphilic molecule: it has a hydrophobic and a hydrophilic part of molecule, In water, the LPS can aggregate. This aggregation process has been followed in our experiment by calorimetry. When we mixed the LPS extracted from Shigella sonnei Re 4350 with different detergents (polymyxin B (PmB), deoxicholat (DOC)) in different concentration, we could diminish the measure of aggregation. The heat treatment process was reversible within the temperature range and detergent concentration used by us.