Two proteolytic preparations, A and F, identified in a previous study to yield casein hydrolysates with the highest bioactivity in Jurkat T cells, were used to hydrolyse sodium caseinate to determine if the bioactive effects of resultant hydrolysates (A1-A5 and F1-F5) were influenced by degree of hydrolysis (DH). None of the hydrolysates (0.25-10.0%, v/v) adversely affected membrane integrity. However, A4 significantly (P< 0.05) reduced Jurkat T cell viability at all concentrations and F5 decreased cell viability at concentrations exceeding 2.50% (v/v). All A hydrolysates, except A4, significantly affected both glutathione (GSH) content and cellular catalase activity at 0.5% (v/v). The effect of F hydrolysates on GSH content were variable. All samples apart from A4 significantly increased catalase activity. None of the casein hydrolysates significantly affected (P < 0.05) ConA-stimulated interleukin-2 production; however, A2 and A3 increased interleukin-10 production. While hydrolysis is considered necessary to release bioactive peptides, no direct bioactivity-DH relationship was observed. (C) 2011 Elsevier Ltd. All rights reserved.