otorhinolaryngological (22 letters),
immunoelectrophoretically (25 letters),
psychophysicotherapeutics (25 letters),
thyroparathyroidectomized (25 letters),
pneumoencephalographically (26 letters),
radioimmunoelectrophoresis (26 letters),
psychoneuroendocrinological (27 letters)
hepaticocholangiogastrostomy (28 letters),
spectrophotofluorometrically (28 letters),
pseudopseudohypoparathyroidism (30 letters).
And..
aequeosalinocalcalinoceraceoaluminosocupreovitriolic (52 letters)