Against a background of growing concern over the health risks associated with exposure to natural sources of radiation, a study has been carried out in Kenya in which the activity concentrations of the major radionuclides in some natural building materials are determined. Different types of rock and soil samples were analysed with a gamma ray spectrometer. Typical activities, so far encountered, are in the ranges: 50 to 1500 Bq.kg-1 for 40K; 5 to 200 Bq.kg-1 for 226Ra; and 5 to 300 Bq.kg-1 for 232Th. The external gamma ray absorbed doses in indoor air, and the corresponding effective dose equivalents in a typical dwelling are presented. The need for further studies is also discussed.