There is a genetic difference in rat hemoglobin (Hb) beta-chain structure, with alternate alleles, A and B, at a single locus. This study was designed to find out whether marrow sensitivity due to gamma exposure and experimentally induced anemia in age-matched adult rats is entirely strain specific or is a combination of both strain and Hb genotype. Eight strains of inbred and outbred rats comprising AA and BB types were examined. The data indicate that there is a relationship between marrow sensitivity and Hb genotypes in response to erythropoietic stress caused by three techniques.