The effect of levamisole on T lymphocyte colony-forming cells (TL-CFC) is determined by the numbers or function of plastic-adherent mononuclear cells in the culture. Using levamisole in cultures of TL-CFC from patients with aplastic anaemia, we have detected a response which is not shown by cells from healthy individuals. Normal TL-CFC produced a biphasic dose-response curve made up of an initial reduction and a secondary return to control values. Similar results were obtained when plastic-adherent cells were treated with the drug before being incorporated into cultures of adherent cell-depleted, untreated TL-CFC. In contrast, colony yields were enhanced by levamisole if plastic-adherent cells were removed before culture or treated, non-adherent TL-CFC were cultured with untreated adherent cells. Cells from aplastic patients contained few TL-CFC but their numbers were enhanced by levamisole and the dose-response curves were similar to those of adherent cell-depleted normal cells and those seen when TL-CFC, but not adherent cells, were exposed to the compound. However, when normal adherent cells were added to cultures of patients' cells, the response resembled the normal biphasic curve. The results suggest that certain T cell functions may be compromised in aplastic anaemia as a result of monocyte dysfunction.