AbstractOverexpression of the multidrug resistance gene,mdr1, and its product, P‐glycoprotein (Pgp), has been associated with cross‐resistance to structurally unrelated compounds in cell lines and tumours. Recently, a non‐Pgp‐mediated form of drug resistance has been described, due to the overexpression of p110, a transport protein. Thirty formalin‐fixed, paraffin‐embedded neuroblastoma samples from 21 cases were examined for overexpression ofmdr1and Pgp using newly established non‐radioactivein situhybridization and sensitive immunocytochemical techniques. Tumours were examined from patients with all the stages of disease and from primary and metastatic sites. Paired tumour samples (pre‐chemotherapy and post‐chemotherapy) were available from cases with stage 2 (n=1) and stage 4 disease (n=8). Immunoreactivity to p110 was also tested on all the samples.Mdr1mRNA was expressed in 16/21 cases and in all the stages. Pgp immunoreactivity was detected in all the cases. Weak cytoplasmic immunoreactivity to p110 was seen in the ganglion cells in 12/21 cases. The expression ofmdr1, Pgp, and p110 showed a statistically significant (two‐sided Fisher exact test,P=0.04, 0.03, 0.04, respectively) correlation with differentiation (Beckwith and Martin grading) but there was no correlation with survival. Pgp immunoreactivity also showed a significant correlation with favourable clinical variables: age less than 1 year at diagnosis and stages 1, 2, and 4 s (two‐sided Fisher exact test,P=0.01