Although studies of drug uptake by leukocytes use similar methods, the results reported are sometimes vastly different. To determine the possible reasons for this, we studied neutrophil uptake of [3H]-clindamycin using the most frequently used density gradient centrifugation method and the volume probes 3H20 and [3H]-polyethylene glycol. We found that the [3H]-clindamycin available to investigators had undergone ra-diolytic decomposition; thus, its microbiologic activity was only 20% of its original potency. By thin-layer chromatography and autoradiography, four extra label-bearing regions were observed with [3H]-clindamycin. Results of neutrophil uptake studies have shown a drop of cellular: extracellular concentration ratios from 40:1 in 1981 to 10:1 in 1982 and 1987.