We investigated the degree of lot-to-lot reagent variation for 5 common immunoassay items. We measured the commercial as well as in-house controls for α-fetoprotein (AFP), ferritin, CA19-9, quantitative hepatitis B surface antigen (HBsAg), and hepatitis B surface antibody (anti-HBs) 10 times each by using both the old and the new lot of reagents whenever a reagent lot was changed, over a period of 10 months. The differences in the mean control values, the percent difference (% difference), and the difference to between-run standard deviation ratio (D:SD ratio) between successive lots were calculated. The % difference in mean control values between 2 reagent lots ranged from 0.1 to 17.5% for AFP, 1.0 to 18.6% for ferritin, 0.6 to 14.3% for CA19-9, 0.6 to 16.2% for HBsAg, and 0.1 to 17.7% for anti-HBs except negative controls of HBsAg and anti-HBs. The maximum D:SD ratios between 2 lots were 4.37 for AFP, 4.39 for ferritin, 2.43 for CA19-9, 1.64 for HBsAg, and 4.16 for anti-HBs. Thus, we have experienced extensive variability in lot-to-lot reagent variation for 5 immunoassay items, indicating that reagent lot-to-lot comparability tests should be continuously performed and that laboratories should determine their own acceptance criteria for each item.