An ultrastructural study of sinus histiocytosis (SH) in axillary lymph nodes was undertaken in 20 patients with primary cancer of the breast. The lymph node responses were first classified by means of light microscopy, according to previously established criteria. The ultrastructural findings established three major patterns of SH response in cancer-draining lymph nodes: developing SH, positive SH, and degenerative SH. These patterns followed closely the classification obtained by light microscopy. An important finding of electron microscopy was the presence of early activation of histiocytes (developing SH) in microscopy (negative SH). The striking ultrastructural resemblance of histiocytes in positive SH to epithelioid cells seen in delayed hypersensitivity reactions (TB) again points to the fact that the positive SH pattern represents a tumor-host reaction of the delayed hypersensitivity type. Finally, although the presence of virus has been shown to play a role in spontaneous BALB/c murine breast tumor, no virions could be found in human cancer-draining lymph nodes.