When human neuroblastoma cells (SH-SY5Y) were exposed to 0.5 - 5 mM acrylamide for 18 hr, the levels of heat shock proteins (HSPs) of 90, 70 and 27 kDa (Hsp90, Hsp70, and Hsp27, respectively) were elevated in the incubation media depending on the dose of acrylamide whereas only the Hsp70 level increased within cells. U0126, a specific inhibitor of extracellular signal-regulated protein kinase kinase and a potent suppressor of the cytotoxicity of acrylamide, suppressed the increase in the levels of all HSPs in the incubation media but not their expression within cells. Total protein concentrations in the incubation media increased depending on the dose of acrylamide, and this increase was associated with the increasing number of bands detected by silver staining after SDS-polyacrylamide gel electrophoresis. One of the clearest bands was identified as Hsp90 by peptide mass fingerprinting. Thus, acrylamide causes release of proteins, including that of HSPs, from SH-SY5Y cells. HSP in extracellular fluid may be a good indicator of cytotoxicity of acrylamide.