It has been found that the lipase QL from Alcaligenes sp. presents a tendency to form very strong bimolecular aggregates (as shown by gel filtration experiments). The addition of detergents (e.g., Triton X-100) is an easy way to break this aggregate. Soluble enzyme in absence of Triton (that is, forming a dimer) was more stable than the enzyme in the presence of Triton. The lack of Triton effect on the stability of immobilized preparations of monomeric enzyme suggests that its main effect is the breakage of the aggregate. The enzyme was immobilized on supports activated with glutaraldehyde in the presence and absence of Triton X-100, to immobilized monomer, or dimers, respectively, and we have found that the properties of the immobilized preparations were very different (after exhaustive washing to eliminate the remaining Triton). When the enzyme was immobilized under conditions where the enzyme tended to form aggregates, stability was much higher, activity was also higher, and the specificity and enantioselectivity of the enzyme were quite different than when the enzyme was immobilized in the presence of Triton. The addition of detergent to the enzyme preparation produced in absence of Triton promoted a release of around 50% of the protein to the supernatant. This suggested that we can immobilize the dimer or the monomer depending on the immobilization conditions. Thus, the control of the lipase-lipase interaction during immobilization dramatically alters the final biocatalyst properties. (c) 2005 Elsevier Inc. All rights reserved.