Clinical data on 49 patients with chronic idiopathic neutropenia (CIN) and 42 patients with neutropenia secondary to a well-defined immunological disorder (SN) were collected and related to serological parameters. In 47% of the patients with CIN and 53% of those with SN, a positive direct immunofluorescence test was obtained with granulocytes from the patients. In the sera from the patients in the two groups, antibodies against donor granulocytes were detected by the indirect immunofluorescence test, the leucoagglutination test and/or the granulocytotoxicity test in 15%, 19% and 15%, respectively. The results of the above tests could not be correlated with any clinical or haematological parameter. Immune complexes in the serum were detected by the 125I-Clq-binding test in 29% of patients with CIN and in 58% of those with SN. The presence of serum immune complexes correlated well with the existence of a low neutrophil count, but not with the presence of recurrent infections, with bone-marrow abnormalities, or with positive reactions in other serological tests. The sera of eight out of 14 patients with CIN and seven out of 12 patients with SN had inhibitory activity for myeloid colony formation in vitro (CFU-GM). This CFU-GM inhibitory activity was correlated with the presence of recurrent infections and with hypoplasia of the myeloid compartment of the bone marrow, but not with positive reactions in other tests. We conclude that the 125I-Clq-binding test probably detects circulating immune complexes that induce a shift neutropenia, whereas serum activity inhibitory for CFU-GM possibly relates to clinically more serious forms of neutropenia. The significance of neutrophil-bound Ig and granulocyte-reactive antibodies in the serum is not clear.