Nuclear expression of c-myc and c-fos oncoproteins was examined by two separate techniques. Flow cytometry revealed that neutrophils from normal (n = 15) and CGL patients before IFNα therapy (n = 10) exhibited similar levels of both oncoproteins. There was no significant alteration in either oncoprotein when CGL patients were treated with IFNα (n = 8). Using a peroxidase method on cytospin preparations from these patients, the pattern of c-myc oncoprotein expression during myeloid maturation was similar in all three groups. It is concluded that the response of CGL to IFNα therapy is probably independent of c-myc and c-fos oncoprotein expression.