The diagnostic potential of oxytalan fibre demonstration in differentiating between leiomyomas and spindle-cell malignant melanomas of the iris was investigated. It was found that oxytalan fibres were abundant in leiomyomata, both between and around the tumour cells, whereas they were found in small numbers only and usually near the iris muscle in malignant melanomata. Their presence and distribution, therefore, appear to offer a satisfactory method of differentiating between these tumours. Since the human choroid and ciliary body normally contain oxytalan fibres, the above findings are not relevant to malignant melanoma of these structures. Naevi and regressing aggregates of iris melanoma cells away from the main tumour mass may similarly be surrounded by misleading amounts of these fibres.