Denatured and fully reduced lysozyme and ribonuclease A were refolded in batch and also in fed-batch operation. In fed-batch refolding of lysozyme, denatured lysozyme was gradually added into refolding buffers containing urea in the concentration range from 1.0 to 2.0 mol/l. The recoveries of the lysozyme activity in a fed-batch operation were higher than those in a batch operation. In the case of ribonuclease A, high recoveries were obtained in both batch and fed-batch operations. The recovery of the enzyme activity in a fed-batch system was estimated from the values of the recovery in a batch operation.