Seventeen patients with suspected drug-induced neutropenia were referred to our laboratory for investigation within a 10-year period. In each case, the suspected drugs were incorporated separately into in vitro cultures of the patients' bone marrow. The cultures were performed in triplicate, using multiple controls. In 10 of these patients a drug-induced inhibition of CFU-C was demonstrated in vitro. The in vitro culture technique is a valuable investigation in patients with suspected drug-induced neutropenia, as it can help identify the causative agent, especially in cases of multidrug administration. It also has a useful application in allowing the clinician to predict drugs to which a patient may be unduly sensitive, and prescribe accordingly.