A delta and C fibre function has been investigated in 18 patients affected by neuropathies associated with monoclonal gammopathies. Warm, cold and heat pain thresholds have been determined by means of a Somedic Thermotest, operating on the Peltier principle, in a room at constant temperature. Our results indicate that: 1) there is an involvement of A delta and C fibres in these neuropathies that might be difficult to quantitate only on clinical grounds 2) in IgG and IgA patients, all affected by a subclinical or mild axonal neuropathy, small fibres of both types are always involved 3) in IgM anti-MAG positive severe cases, A delta fibre involvement, suggestive of demyelination is evident, while in one anti-MAG negative patient only C fibres are damaged, probably reflecting a different pathogenetic mechanism.