Nine cases of malignant lymphomas of undifferentiated type, which is difficult to classify whether they are derived from reticulum cell or lymphocyte by light microscopy, were collected and studied by electron microscopic and histopathological examinations, and calssified into two types from electron microscopic findings. Some characteristic features, including cellular size and shape, and quantity and quality of cytoplasmic and nuclear structures at the electron microscopic level, were compared between the two types of lymphomas with similarity to reticulum cell and to lymphoblastic cell. From the aspects of systemic and synthetic view, it is possible to classify whether these tumor cells correspond to the so-called reticulum cell or to lymphoblastic cell in malignant lymphomas of undifferentiated type, which was previously thought to be indistinguishable. In addition to these results, clinico- and histo-pathological analyses were also made, with short discussion.