The production of L-3,4-dihydroxyphenylalanine from mushroom tyrosinase immobilized on chemically modified supports, sodium aluminosilicate (NaA) and calcium aluminosilicate (CaA) (two separate forms of zeolite) was studied. Tyrosinase was immobilized on the supports using glutaraldehyde (1 %, w/v) as a cross-linking agent. The uptake of tyrosinase onto calcium aluminosilicate was comparable to that with sodium aluminosilicate (85% versus 82%). The behavior of the immobilized enzyme and the strength of the enzyme attachment was studied. There was no loss of activity of the immobilized enzyme during 40-48 h of repeated batch operations. Thus, the stability of tyrosinase on these supports is much superior to the stability of the enzyme when immobilized on other supports. L-DOPA production rates during a 7 h batch reaction using immobilized tyrosinase was found to be in the range of 31-36 mg1(-1) h(-1). The results of these studies indicate that L-DOPA production using tyrosinase immobilized on NaA or CaA is feasible.