The frequency of binucleated cells containing one or more micronuclei (MNBN cells) in cytokinesis-blocked peripheral blood lymphocytes can be used to determine whether a person has received an overexposure to ionizing radiation. However, the absence of a pre-exposure sample can preclude precise dosimetry. Here we use a database of MNBN cell frequencies in peripheral blood lymphocytes from 3,104 apparently healthy, unexposed, control subjects aged birth to 88 years, contributed by laboratories participating in the HUMN project. To determine whether a putatively exposed person has actually received a measurable dose, that person's peripheral blood lymphocyte MNBN frequency is compared to values from age and gender-matched controls in the database. If the subject's frequency is significantly higher than the controls, then a cobalt-60 dose–response curve obtained with the cytokinesis-block micronucleus (CBMN) assay in human peripheral blood lymphocytes is used to estimate the minimum dose of low-LET radiation that could have caused the increase. The response curve was generated with 11 acutely administered doses ranging from 0–4 Gy; the majority of doses were in the low end of this range to provide an accurate estimate of the linear portion of the response. The minimum detectable acute whole-body dose at the 95% prediction level and their corresponding 95% confidence intervals are 0.18 Gy (0.15–0.22) and 0.20 (0.17–0.24) Gy for 20-year-old males and females, respectively. Corresponding values for 50 year olds are 0.23 Gy (0.19–0.26) and 0.25 (0.21–0.29) Gy, and for 70 year olds are 0.24 (0.21–0.28) Gy and 0.26 (0.22–0.31) Gy. The minimum detectable chronic doses are approximately fivefold higher for both genders. These types of analyses, including knowledge of assay variability, will improve our understanding of the requirements and limitations for biodosimetry when a pre-exposure micronucleus value is unavailable and reliance on historical baseline micronucleus values is required.