An enzyme-linked immunosorbent assay (ELISA) for rat α2-macroglobulin (α2M) was developed, and the concentrations of α2M in sera in normal Crj:CD and Crj:WI rats as well as in turpentine oil injected Crj:WI rats were determined.Anti-rat α2M serum was prepared by immunization of rabbits with the α2M fraction separated from rat acute phase serum using Sephacryl S-300. The specificity of the rabbit anti-rat α2M serum was confirmed by immunoelectrophoresis and Western blotting.The α2M concentrations in 180 normal Crj:CD (90 males and 90 females) and 180 Crj:WI (90 males and 90 females) rats were 14.5–94.9 μg/ml (mean 36.8 ± 13.3) and 13.6–65.8 μg/ml (mean 34.9 ± 12.4), respectively. There were no significant differences between male and female Crj:CD and Crj:WI rats, respectively (p < 0.01). No significant age-related difference was found in Crj:CD or Crj:WI rats (p < 0.01). The α2M concentrations in sera from 25 Crj:WI rats 48 h after inoculation of turpentine oil were 440–6498 μg/ml (mean 2621 ± 1727).