The radioactive concentrations of 18F, 88Y and 152Eu solutions and the activity of 222Rn gas ampoules are measured using a primary method involving 4πγ NaI(Tl) integral counting with a well-type NaI(Tl) detector and efficiencies computed by Monte Carlo simulations. The simulations use the GEANT code coupled with a routine (sch2for), which generates randomly the decay paths and emissions depending on the decay scheme parameters. The resulting radioactive concentrations of 88Y, 152Eu and 18F are found to agree with those measured with other primary measurement methods, such as 4π (β, e, X)-γ coincidence counting or liquid scintillation counting. Results of the determination of the activity of 222Rn gas ampoules by this method also match the results of an absolute standardisation technique in which radon is condensed onto a cold surface and its α-emissions are detected through an accurately specified solid angle.