Introduction A mouse melanoma discovered and described by Harding and Passey (1) was brought to our attention by workers with this tumor at the Memorial Hospital of New York.2 Attempts to grow it in tissue culture were at first unsuccessful. Harding and Passey state that such attempts, made at their instigation, “had failed to give results which would satisfy them that growth of tumour cells had actually been obtained.” In experiments made in this laboratory it was found that fragments taken from heavily pigmented tissues of the Harding and Passey melanoma markedly increase the alkalinity of the culture medium, in which case tissue growth is inhibited. By a method to be described here, it has been found possible to obviate this condition and to obtain good growth. This paper presents a description of the various types of cells which appear in the outgrowth. Melanoma appears to be closely associated with the epidermis and involves the subcutaneous tissues. The characteristic cell seems to be a highly dendritic cell, called the melanoblast or pigment-producing cell. Becker (2) has given an extended review of the literature on the origin of the pigment-producing cells in the skin and buccal mucosa. The epithelial origin claimed by Bloch (3) for the dendritic melanoblasts has been questioned by others, who consider that it arises from dermal cells or from some part of the peripheral nervous system (Masson, 4). All agree, however, that the typical cell of the melanoma is a dendritic, melanin-producing cell whether it originates from normal melanoblasts or from other cells. Associated with this melanin-producing cell in the tumor may be other cells heavily laden with pigment granules which are generally considered to be phagocytic. In sections of melanomas, the cell bodies of the neoplastic melanoblasts have been classified as fusiform or cuboid and the tumors accordingly termed sarcomatous or carcinomatous. Not much reliance can be placed on mere shape (cf. Becker, 2). The term chromatophore is descriptive, meaning a pigment-carrying cell. It does not necessarily imply that the cell possesses the property of synthesizing pigment. For this reason the term is not used in this paper.