This report describes the development of a potentially clinical method to measure the cellular metabolites of zidovudine (ZDV) in patients receiving the drug, This new method combines the use of Sep-Pak cartridges to separate ZDV phosphates with radioimmunoassaying to quantitate ZDV, The detection limit is 0.02 pmol/10(6) cells, and this assay can measure a wide range of intracellular drug concentrations, The use of the cartridge-radioimmunoassay methodology should prove very useful for in vivo cellular pharmacokinetic studies of ZDV.