20 patients with aplastic anaemia were treated with methylprednisolone 1 g/d for 3 d followed by prednisolone 60 mg on alternate days. At 3 months after therapy, 7 of 20 patients (35%) showed recovery, 2 had died and 11 were non-responders. Only newly diagnosed patients were responders. Results of in vitro tests were correlated with clinical response. The first test was designed to determine whether removing various subpopulation of cells would increase the numbers of progenitor cells. In the second test blood cells were collected at 1 month after starting treatment and assayed for progenitor cells. Data from both tests did not correlate with the response to therapy. Lymphocyte subsets were measured in the blood before and at 1 month after treatment. The absolute numbers of OKT3+, OKT4+ and OKT8+ cells were significantly decreased compared to the normal population. The number of HNK1+ cells was also decreased but no significant difference was observed. Responders had a higher number of HNK1+ cells compared to non-responders. No significant differences of OKT3+, OKT4+ and OKT8+ cells among responders and non-responders were observed. Therefore a high number of HNK1+ cells prior to therapy may identify patients responding to the treatment.