Lymphocytic and plasmacytic infiltration surrounding a malignant tumour probably represents an immunological response of the host directed against the neoplasm. Lymphocytic infiltration has a favourable effect on the prognosis in a number of human tunours, including cutaneous melanoma. The present study includes 326 consecutive cases of cholroidal melanoma which were examined for cellular infiltration. These data were then correlated with other histological parameters and with prognosis. An intense cellular infiltration was found in fifteen tumours (4.6 per cent) and a moderate infiltration in forty (12.3 per cent). Tumours with cellular infiltration were significantly larger and more vascularized than the remaining tumours, which may relate to the accessibility of the immune system to the tumour. There was a higher percentage of poorly differentiated tumours among those with cellular infiltration. However, it does not appear that cellular infiltration favourably influences the prognosis of choroidal melanoma, as 5-year survival rates did not correlate with the degree of cellular infiltration demonstrated by the primary tumour. Although choroidal melanomas are capable of inciting an immune response, this response is apparently ineffective in preventing metastastic spread.