Since 1985, the International Commission on Radiological Protection has revised biokinetic models for dose calculations from intakes of radionuclides. For workers and reference persons of the public, these models include recycling exchanges of the activity between the mixed blood and soft tissue pool and the other organs. Current work is aimed at providing a calculation tool to solve such recycling systems based on matrix algebra. The software, named CYCLOMOD, is written in Visual Basic language using Excel5. Database files provide radionuclide compartment transfer parameters corresponding to adults and children. CYCLOMOD processes any combination of acute and/or chronic intake by inhalation, ingestion and injection, for a chosen monitoring period of time following intake. Output files provide radionuclide remaining quantities, disintegrated amounts and number of disintegrations in each compartment as a function of time after intake. For each compartment, tissue and organ data are also presented graphically and thus could be related to in vivo organ measurements. The daily faecal and urinary excretions can also be calculated by subtraction.