Cells from an established human neuroblastoma cell line, SH-SY5Y, were demonstrated to grow and form solid tumours in nude rats. This cell line, which is an adrenergic subclone of the SK-N-SH cell line, has previously been used in differentiation model studies. The tumours retained the neuronal phenotype of the cultured cells, as evidenced by the expression of neuron-specific enolase (NSE) and chromogranin A + B. The transcription factor Isl-1, a protein expressed in subsets of neurons and endocrine cells as well as in neuroblastoma cells, was also expressed in the transplanted tumours, thus further verifying the retained phenotype of the cells under in vivo conditions. At scintigraphy utilizing 123I-MIBG the optimal tumour/background ratio was obtained 20 h after injection. The assessment of tissue/serum ratios showed the highest uptake in the spleen (0.067% per gram of inj. activity), neuroblastoma tumours (0.067% per gram of inj. activity) and in the adrenals (0.065% per gram of inj. activity).