Variation in the amounts of radioactivity that is associated with dose calibration and administration of gamma-emitting radioactive drugs was studied. Health systems use radionuclide dose calibrators when they need to assay drugs for radioactivity. However, the radioactive drugs commonly used for therapeutic or diagnostic purposes are compounded and assayed in or administered from containers that differ from those containing the standard reference materials (SRMs). SRMs for four radionuclides--technetium 99m, indium 111, thallium 201, and iodine 131--were drawn up into vials and syringe-needle assemblies of volumes and sizes represented in clinical practice. In each sample, the amount of radioactivity was calculated and compared with values obtained from three dose calibrators. In addition, over a four-month period, 101 samples of technetium Tc 99m medronate injection with a desired activity of 20 mCi were prepared in syringes; the radioactive dose in each syringe was calibrated for administration to a patient at a specific time that day. The amounts of radioactivity at the time of preparation and the time of administration, the amount remaining in the syringe and needle after administration, and the amount reported as administered were recorded. Measurement with the dose calibrators of the SRMs in containers supplied by the National Institute of Standards and Technology showed radioactivity within 10% of the labeled amount, the percentage of variation regulations allow. Measurements of the SRMs in syringe-needle assemblies were within 10% for technetium 99m and thallium 201, 9-16% for iodine 131, and 15-26% for indium 111. The individual doses of technetium 99m medronate injection were, on average, administered on time, but doses were administered up to 75 minutes before and 107 minutes after the calibration time. The mean +/- S.D. amount administered was 19.0 +/- 1.34 mCi. The mean +/- S.D. amount reported administered was 20 +/- 0.24 mCi. How radiopharmaceuticals were dose calibrated and administered influenced the actual dose available to patients.