An immunoperoxidase technique has been applied to the detection of intracellular immunoglobulins at the light and ultrastructural levels in three untreated cases of Hodgkin's disease. The results are compared with those obtained in three treated cases. In both groups, 20-90% of malignant cells had intra-cytoplasmic immunoglobulins. There was no correlation between the percentage of immunoglobulin-containing cells and the histological type or the stage of the disease. At the ultrastructural level, immunoglobulins were constantly localized on cytoplasmic ribosomes, the later being either free in the cytoplasm or bound to the endoplasmic reticulum and to the external envelope of the perinuclear space. In addition, a very few malignant cells exhibited immunoglobulins within their perinuclear space and their endoplasmic reticulum. These results demonstrate that immunoglobulins in Hodgkin's malignant cells are present on the cellular sites of protein synthesis. They appear to be retained in their cytoplasm, and to be secreted only very occasionally. The significance of these findings as to the cellular origin of malignant cells in Hodgkin's disease is briefly discussed.