The programme and results of internal exposure monitoring of four individuals who have worked for 7 to 10 years in a 131I radiopharmaceutical production laboratory are described. The monitoring has been carried out using an in vivo thyroid measurement technique. The maximum and minimum average annual intakes of 131I received by the laboratory workers was 536 and 79 kBq, respectively. The highest estimated annual intake of 131I, incurred by one of the workers in a single year, was nearly 3.8 MBq. The mean values of the annual intakes by all the workers in their working periods in the laboratory was below the intake limits recommended by the ICRP.