The aim of the present study was to assess the innervation pattern in benign haemangiomata, malignant colo-rectal carcinomata and experimental malignant tumours transplanted into severe combined immunodeficient (SCID) mice using the general nerve fibre marker protein gene product (PGP) 9.5. An indirect immunohistological technique (using streptavidin-biotin complex formation) was used on paraffin wax sections of tumour tissues fixed in neutral buffered formalin. In 5/7 haemangiomata nerve fibres were detected in the vicinity of some tumorous blood vessels, though the majority of such vessels were without immunoreactive nerve fibres. In some cases the reaction product for PGP 9.5 appeared to be distributed in a diffuse perivascular manner. In the colo-rectal carcinomata, no intra-tumorous blood vessels received a nerve supply, though nerves were observed in the stroma supporting the tumour tissues. In the experimental tumours derived from either human colo-rectal carcinomata or breast carcinomata cell lines, no nerve fibres were detected within the tumours themselves. The conclusions reached were that tumour blood vessels are not innervated. Angiogenesis, which is a prerequisite for tumour growth, must, therefore, be regulated by a means other than neural.