Neutralizing anti-IFN alpha antibodies (nIFN alpha Abs) occur in a significant proportion of patients with hairy cell leukaemia, hepatitis or solid tumours treated with recombinant IFN alpha (IFN alpha 2a or IFN alpha 2b), but information on their incidence in chronic myeloid leukaemia (CML) is scanty and their clinical relevance is not yet completely defined. By using an IFN alpha antiviral neutralization bioassay, the frequency of nIFN alpha 2a Abs was evaluated in 67Ph+ CML patients during IFN alpha 2a therapy at doses ranging from 6 to 9 MU/d. 15 patients (22%) developed nIFN alpha 2a Abs (titre ranging from 1:40 to 1:20480) and 11/15 (73%) were haematologically and/or karyotypically unresponsive to treatment. 52 patients did not develop antibodies and 11 of them (21%) were unresponsive. The negative relationship between the positivity for nIFN alpha 2a Abs and the response to treatment was highly significant (P = 0.0001). In nine nIFN alpha 2a Abs positive patients, treatment was changed from recombinant IFN alpha 2a to lymphoblastoid IFN alpha (IFN alpha-ly), at the same dose and schedule. After 9 months of IFN alpha-ly treatment a haematological response was achieved in 4/7 cases who were non-responsive to prior IFN alpha 2a therapy and was maintained in the other two patients previously responsive to IFN alpha 2a. However, no karyotypic response was observed. This data shows that a significant proportion of Ph+ CML patients receiving treatment with IFN alpha 2a can develop neutralizing antibodies and that these antibodies are associated with a loss of IFN alpha 2a efficacy. Changing the patients to treatment with lymphoblastoid IFN alpha may restore haematological response but it is not likely to induce a karyotypic response.