In this work, a recently developed mathematical model of the lymphocytopoietic system in acutely irradiated humans was extended to predict the dynamics of this system both in nonirradiated and acutely/chronically irradiated humans. The mathematical implementation of this model is a system of nonlinear ordinary differential equations, whose variables and parameters have clear biological meaning. We demonstrate that the model is capable of reproducing the dynamic regimes that are typical for lymphocytopoiesis in nonirradiated individuals with a hematological disorder (cyclic lymphocytopenia) and in patients receiving allogeneic stem cell transplantation. The model is also capable of predicting the dynamics of the lymphocytopoietic system in humans exposed to acute and chronic irradiation over a wide range of doses and dose rates. Additionally, the “lethal” dose rate of chronic irradiation, evaluated in the framework of the lymphocytopoiesis model, agrees with the actual minimum dose rate of lethal chronic irradiation observed for humans.