The mean percentage of CD34 + cells expressing VLA-4 in the BM and PB was relatively high (73.4% ± 12% and 65.4% ± 6.6%, respectively) and dropped considerably in the PBSC collections to 43.5% ± 7.1% with a similar pattern observed for patients in arms A, B, and C. However, when the patients were regrouped as "good mobilizers" and "poor mobilizers," a higher percentage of CD34 + cells expressing VLA-4 was observed in the PBSC of the pooled "good mobilizers" (50.5% ± 9% versus 36.3% ± 6.4%; p = 0.01). We conclude that release of CD34 + cells to the PB involves a general downregulation of Thy1, L-selectin and VLA-4 on CD34 + cells, irrespective of the growth factor used for mobilization. However, good mobilizers had a relatively higher percentage of CD34 + cells expressing the VLA-4 antigen.