Samples of human rib were collected at autopsy and 20 were selected for marrow fat cell measurement, representing an age range of 16-96 years. The mean diameter of fat cells in red bone marrow of human rib was found to increase from around 48 microns at ages 16-29 years to around 65 microns at ages 82-96 years. There was a greater number of fat cells of smaller size range in younger ages compared with that in older ones. The maximum size of fat cells was found to be 102 microns. Calculated radon-derived doses to haemopoietic tissue ranged from 60 to 162 microSv y-1 at average UK exposures of 20 Bq m-3. It was concluded that the bone marrow fat fraction is the important parameter as far as alpha-radiation dose from radon in fat is concerned. This updates the theoretical estimates of dose carried out by Richardson et al. (1991).