Serum IgE levels were measured (radioactive radial diffusion or radioimmuno assay) in 21 patients (14 mycosis fungoides or premycosis fungoides and 7 other lymphomas) and 1,019 controls. Elevated IgE levels (greater than or equal to 500 UI/ml) were found in 57 p. 100 of patients and 8,1 p. 100 of controls. The most important increase was noted in mycosis fungoides. Moreover, some of these patients showed a cellular immunity impairment: negative delayed skin tests, low percentage of T-lymphocytes forming E rosettes (2 out of 4 patients), decreased mitogenic response to Con A and P.H.A. In one patient with Sézary syndrome there was a dissociation between E and E active rosettes (low values) and anti H.T.L.A. serum (high values). This result could indicate that the Sézary cell is a poorly differentiated T-lymphocyte.