In vitro studies of marrow growth from patients with AA have generally confirmed the hypothesis that the disease is due to stem cell injury. By studies of marrow growth from AA patients (when possible) and their relatives to detect altered sensitivity of marrow stem cells to drugs or toxins, it may be possible better to identify suspected aetiological agents, and study genetic susceptibility to toxic AA. Lack of adequate stimulation has not been shown to be aetiological in AA as both ESF and CSF levels are generally higher than normal. Serum from AA patients has not been shown to be toxic to marrow and recent data suggesting that lymphocytes mediate some cases of AA have been questioned. Further studies of committed stem cell growth and the interaction between lymphocytes and stem cells are indicated in attempting to understand the complex of diseases that result in marrow aplasia.