We have studied the ultrastructural organization of nucleoli in benign naevi and malignant melanomas. In benign naevus cells the nucleoli displayed a compact ribonucleoprotein distribution, with one or two large fibrillar centres. In malignant melanoma cells the nucleoli were large with an irregular, nucleolonema-like ribonucleoprotein distribution and they exhibited numerous, small fibrillar centres. Statistical evaluation of the size of fibrillar centres indicated a mean value of 0.482 micron 2 +/- 0.136 SD for naevi and 0.221 micron 2 +/- 0.128 SD for malignant melanomas. These features, together with the more dispersed chromatin pattern of malignant melanoma nuclei compared with those of benign naevus cells, are proposed as diagnostic parameters which differentiate benign naevi from malignant melanomas at the ultrastructural level.