Buffy coat preparations of peripheral blood from two patients with Stage IVB Hodgkin's disease, 33 patients with Hodgkin's disease undergoing complete staging including laparotomy, 12 normal controls and eight patients with viral upper respiratory infections were examined for the presence of abnormal cells. Primitive histiocytic cells were seen in the two patients with IVB disease and in two patients with IIIB disease all of whom proved to be refractory to chemotherapy. Reed-Sternberg cells were seen in one IVB patient. Large cells with basophilic, agranular cytoplasm and moderately convoluted nuclei often with a perinuclear halo and nucleoli were detected in 30 of 35 patients with Hodgkin's disease, five of eight patients with viral infections but in none of 12 normal controls. Other abnormal large cells with round nuclei, occasional prominent nucleoli, grey to lightly eosinophilic cytoplasm and diffuse granulation were seen almost exclusively in Hodgkin's disease. There was no correlation between histology, stage, splenic involvement, skin test positivity and response to therapy with the presence or frequency of either of these cell types. This observation suggests that these cells are more likely to be 'reactive' rather than malignant. It is unusual for abnormal histiocytic cells to circulate in Hodgkin's disease and their presence may be a poor prognostic sign. The presence of other types of large atypical cells is not indicative of either haematogenous spread of Hodgkin's disease or of poor prognosis after treatment with radiotherapy alone.