This report is the user's guide of the MiniBIOS model, version 1A4. The model is operational at the Laboratory of Radiation Research of the RIVM. MiniBIOS is a simulation model for calculating the transport of radionuclides in the biosphere and the consequential radiation dose to humans. The transport of radionuclides in the biosphere is described with transfer coefficients between compartments representing soils, river waters, river sediments and marine waters. The radiation dose to humans is calculated via various exposure pathways, including ingestion of water, fish, vegetables, meat and milk, inhalation of suspended soil and external irradiation. The MiniBIOS model is developed by the National Radiological Protection Board (NRPB, England) for use in probabilistic assessments of the radiological impacts in disposing radioactive waste in geological formations. The Laboratory of Radiation Research used the model in the PROSA project, a study on the safety of the disposal of radioactive waste in salt formations.