The transplantation of peripheral blood precursor cells (PBPC) is becoming of interest for autografting patients with a wide variety of haematological and other malignancies. For rapid quality control of PBPC apheresis products, flow cytometry is applied to quantify the number of CD34+ events. We studied the effect of different storage conditions on the number of CD34+ counts in EDTA-anticoagulated aliquots of PBPC grafts. Within 24 h, CD34+ signals decreased when samples were stored at room temperature (RT, 20 +/- 2 degrees C) compared to the results obtained directly after cytapheresis. The signal rate equalled or exceeded the baseline values after 24 h when aliquots were deposited at room temperature and subjected to agitation. Storage at 4 degrees C revealed no significant changes. These data indicate that quality control of PBPC samples by flow cytometry significantly depends on storages time, temperature and other conditions like the agitation of the specimen.