Ficoll-Triosil separated peripheral blood mononuclear (PBM) cells were analysed by fluorescent microscopy with Ortho monoclonal antisera to T cells (OKT3+) helper cells (OKT4+) and suppressor cytotoxic cells (OKT8+) and with polyclonal antiserum to surface immunoglobulin. Twenty-five normal subjects, 16 patients with acute polyradiculoneuritis (Guillain-Barré syndrome, AP) and 10 patients with other neurological diseases were studied. The percentages of OKT3+, OKT4+ and immunoglobulin bearing cells were similar in the three patient groups but the percentage of OKT8+ cells was reduced to 13.5 +/- 4.1 (mean +/- s.d.) compared with 19.4 +/- 4.9 in the normal subjects and 18.5 +/- 3.1 in the patients with other neurological diseases. The ratio of OKT4+/OKT8+ cells was correspondingly increased to 3.5 +/- 2.1 compared with 2.1 +/- 0.5 in the normal subjects and 2.1 +/- 0.4 in the patients with other neurological diseases. In one test of suppressor cell function Con A incubated mitomycin treated PBM cells were added to autologous PBM cells stimulated with Con A to compare the degree of suppression with that produced by control incubated mitomycin treated cells (Con A suppression test). In a second test of suppressor cell function short lived suppressor cell (SLS) activity was assayed by comparing PBM stimulation by Con A added at the start of culture with that produced by Con A added after 24 hr. Neither test revealed any significant differences between AP patients and control subjects.