Normal donor peripheral blood mononuclear cells incubated for 24 h with sera from patients with Hodgkin's disease were investigated by electron microscopy for the presence of parallel tubular structures (PTS) and/or amorphous electrondense granules (large granular lymphocytes = LGL). In comparison with normal human serum, 14 out of 29 sera of the patients induced a marked increase in the percentage of LGL. From a limited number of experiments it was likely that this increase is paralleled by an increase in Fcgamma receptor-bearing cells after the incubation. This serum effect did not show a correlation with the number of Fcgamma receptor-positive lymphocytes in the peripheral blood of the patients. A difference in the induction effect could be demonstrated between the sera from patients with a favourable and those with an unfavourable clinical course, but this distinction was not absolute. The presence or absence of splenic involvement by Hodgkin's disease does not apparently influence the effect of the sera. From experiments using sera positive for immune complexes or anti-Epstein-Barr virus antibodies, it seems unlikely that these factors are responsible for the observed increase in LGL.