To evaluate the feasibility and efficacy of CD34+ cell immunoselection from routine peripheral blood stem cell (PBSC) harvests in very small children a prospective study was performed in 15 children with advanced neuroblastoma weighing 20 kg or less. Products of two consecutive leukaphereses carried out on a COBE Spectra separator after G-CSF alone mobilization were pooled for immunoselection on Ceprate column. The median number of CD34+ cells and total CFU-GM collected were respectively 5.9 x 10(6)/kg (range 2.3-23.4) and 126.9 x 10(4)/kg (range 52.9-559.9). After separation the median number of CD34+ cells in the adsorbed fraction was 2.6 x 10(6)/kg (range 1-9.8) with a median purity of 54% (range 21-82) and a median of 95.7-fold (range 35-250) enrichment. Thirteen patients underwent autografts with CD34+ PBSCs after a busulfan 600 mg/m2 + melphalan 180 mg/m2 preparative regimen. The median number of days to achieve an absolute granulocyte count of 0.5 x 10(9)/l and a platelet count of 20 x 10(9)/l were respectively, 12 (range 10-24) and 35 (range 25-43). The median number of platelet transfusions was nine (range 2-15). We conclude that safe and effective immunoselection and transplantation of CD34+ PBSC can be accomplished in children with low body mass.