SUMMARY.— The skin‐window technique was applied to normal und involved skin sites of 17 patients with mycosis fungoides, 8 patients with parapsoriasis en plaque, and 6 patients with cutaneous lymphomata, and also to normal skin of 25 controls. Large mononuclear cells rapidly migrated in large numbers from plaques of mycosis fungoides and parapsoriasis. Identical cells were seen in skin windows from control sites at later intervals. No malignant cells were found in any of the skin windows of mycosis fungoides or purapsoriasis. Malignant cells were found in cases of malignant cutaneous lymphoma. Comparison of cells found on the skin windows from mycosis fungoides plaques to lymph node imprints indicates that the former are immature reticular cells without neoplastic features. We conclude that mycosis fungoides is a proliferation of these benign immature reticular cells in the skin.