A 29-year-old man presented with a 20-pound weight loss and was found to have a large retroperitoneal mass. Electron microscopy was diagnostic for metastatic amelanotic melanoma, characterized by large tumor cells with prominent nucleoli and focal clusters of membrane-bound, ellipsoid granules (arrowheads in A). Higher magnification of these organelles showed that they were premelanosomes containing perpendicular striated filaments (B and C) or parallel lamellar arrays (D). Extremely electron-dense melanin was absent from the premelanosomes. Immunoperoxidase stains of tumor cells were positive for the melanoma-associated antigen HMB-45 and negative for leukocyte common antigen, B-cell antigen (CD20 and CD19), T-cell antigens (CD2, CD3, CD4, CD5, and CD8), and lymphoid activation antigens (CD30 and CD15). Retrospectively, the patient was stated to have fair skin, numerous freckles, and red hair; no history of excesssive sunburns or skin lesions was elicited. Original magnifications: (A) ×8,000; (B) ×90,000; (C) ×71,500; and (D) ×91,000. (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)