In long-term space travel, the crew is exposed to microgravity and radiation that invoke potential hazards to the immune system. T cell activation is a critical step in the immune response. Receptor-mediated signaling is inhibited both in microgravity and modeled microgravity (MMG) as reflected in diminished DNA synthesis in peripheral blood lymphocytes and their locomotion through gelled type I collagen. Direct activation of Protein Kinase C (PKC) bypassing cell surface events using the phorbol ester PMA rescues MMG-inhibited lymphocyte activation and locomotion, whereas calcium ionophore ionomycin had no rescue effect. Thus calcium-independent PKC isoforms may be affected in MMG-induced locomotion inhibition and rescue. Both calcium-dependent isoforms and calcium-independent PKC isoforms were investigated to assess their expression in lymphocytes in 1g and MMG-culture. Human lymphocytes were cultured and harvested at 24, 48, 72 and 96 hours and serial samples assessed for locomotion using type I collagen and expression of PKC isoforms. Expression of PKC-, - and - was assessed by RT-PCR, flow cytometry and immunoblotting. Results indicated that PKC isoforms d and e were down-regulated by more than 50% at the transcriptional and translational levels in MMG-cultured lymphocytes compared with 1g controls. Events upstream of PKC such as phosphorylation of Phospholipase C (PLC-) in MMG, revealed accumulation of inactive enzyme. Depressed Ca⁺⁺-independent PKC isoforms may be a consequence of an upstream lesion in the signal transduction pathway. The differential response among calcium-dependent and calcium-independent isoforms may actually result from MMG intrusion events earlier than, but after ligand-receptor interaction.