The role of humoral immune mechanisms in the pathogenesis of the neutropenia in five patients with chronic killer (K)-cell lymphocytosis was studied. For the detection of neutrophil antibodies in the patient's blood, immunofluorescence, cytotoxicity and agglutination tests and a sensitive antibody-dependent cellular cytotoxicity (ADCC) assay were applied. An assay for bone-marrow colony growth (CFU-GM, BFU-E) inhibition was applied as well. A C1q-binding test was used to detect immune complexes. The lymphocytes of all five patients had the capacity to lyse alloantibody sensitized neutrophils. Neutrophil-bound IgG was found only in one patient. Two patients had neutrophil-reactive antibodies in their serum; in one patient these antibodies were only detectable in the ADCC using the patient's serum to sensitize target cells. However, these antibodies reacted also with lymphocytes, were absorbable with platelets and thus probably were HLA antibodies. The serum of one of these two patients showed complement-dependent CFU-GM and BFU-E inhibition, but the observed inhibition was probably also due to the anti-HLA antibodies present in his serum, as the inhibiting effect disappeared after absorption of the serum with platelets. The serum of only one patient contained low amounts of immune complexes, as measured in the C1q-binding test. Our data suggest that humoral autoimmune mechanisms, such as autoantibodies against neutrophils or neutrophil precursors or circulating immune complexes, do not seem to play an important role in the K-cell lymphocytosis/neutropenia syndrome. The possible role of the expanded K-cell population, and its humoral products (tumour necrosis factor, interferons), in the syndrome is discussed.