Clustering of lymphocytes around Reed-Sternberg cells was noticed in single cell suspensions made from viable Hodgkin's lymphoid tissue. Cytocentrifugation of the suspension showed that clustering also occurred around a smaller cell type, thought to be the precursor of the classical Reed-Sternberg cell. Time-lapse cine films taken of the clustering showed unceasing activity on the part of the lymphocytes migrating over the surface of the central cell. Reed-Sternberg cells were reacted with anti-monocyte serum using indirect fluorescence techniques. In its mature form at least, the Reed-Sternberg cell showed no activity with the antiserum. No immunoglobulin was detected in the Reed-Sternberg cell using fluorescence techniques, but a few Reed-Sternberg cells showed diffuse cytoplasmic staining using the peroxidase-labelled antibody technique. Membrane receptor tests showed the lymphocytes surrounding the Reed-Sternberg cell to be T-cells. After proteolytic enzyme treatment to free lymphocytes from the surface, the Reed-Sternberg cell bound IgG-coated red blood cells indicating a probable Fc receptor. Cytochemistry demonstrated weak non-specific esterase activity in a small minority of Reed-Sternberg cells, and absence of acid phosphatase, alkaline phosphatase and peroxidase. A subpopulation of lymphocytes with distinctive segmentation of the nucleus was noted. These were often to be seen participating in lymphocyte rosettes around the Reed-Sternberg cell.