A selected series of 669 primary malignant melanomas of the skin, stage I, was studied. The series includes 86 lentigo maligna melanomas, 259 superficial spreading malignant melanomas, 194 nodular malignant melanomas and 130 unclassifiable malignant melanomas. The tumour cell type was classified and the tumour cell pigmentation, the cellular atypia and the mitotic count was graded. The relation of these four tumour cell features to each other and to the tumour type was studied by X2tests. The prognostic value of these features in relation to the total series as well as to each tumour type was also examined. The most common features were mixed cellularity, little pigment, moderate atypia and low mitotic count. Most of these tumours were superficial spreading malignant melanomas. A good prognosis was related to spindle-shaped tumour cells, marked pigmentation, slight atypia and few mitoses. A bad prognosis was related to epithelioid tumour cells, little pigment, marked atypia and many mitoses. Variations of lentigo maligna melanoma tended to be more benign while variations of nodular malignant melanoma tended to be more malignant than the average. A superficial spreading malignant melanoma might vary in either direction.