THE constitutive and heat-inducible expression of HSP105 was investigated in newborn mouse brain cell cultures by Northern blotting, Western blotting and immunocytochemistry. HSP105 was expressed most abundantly in the brain among the various tissues examined. HSP105 mRNA and protein were both present at substantial levels in brain cell cultures under unstressed conditions and up-regulated greatly during 3–4 8h following exposure to heat stress (4 3°C/2 0min). HSP105 was expressed in nearly all neurons, oligodendrocytes, microglia and astrocytes with its location of both cytoplasmic and nuclear regions under unstressed and heatstressed conditions. HSP105 expression was significantly down-regulated in astrocytes following treatment with IL-β or TNF-α (5 0ng/ml for 6 days), both of which are known growth-stimulatory cytokines for astrocytes. These results indicate that HSP105 is constitutive and heat-inducible HSP in neurons and glial cells in which its expression is under the control of both stressful stimuli and growth-regulatory factors.