The humoral immune response to human immunodeficiency virus type 1 (HIV-1) was studied in 25 AIDS patients with CD4 lymphocyte counts of less than 400/mm3. Humoral immune responses against tissue culture adapted strains of HIV-1, and two limited-passage patient isolates were investigated. Total anti-HIV antibody levels were not significantly different between different individuals. Neutralizing titres against HIVLA1 and HIVSF2 were 10- to 100-fold higher than against clinical isolates. The complement-mediated, antibody-dependent enhancement of HIV-1 infection titre was high (mean 1:14,000). Antibody-complement mediated cytotoxicity of both HIVLA1 and HIVSF2 was ineffective using human complement as a complement source. The antibody-dependent, cell-mediated cytotoxicity (ADCC) activity varied against the four isolates with tissue culture-adapted strains being more susceptible than clinical isolates. Finally, an ADCC effector cell function, natural killer or NK activity, was measured for all 25 patients, and NK activity of patients was decreased by nearly 75% compared to uninfected individuals. In summary, beneficial humoral immune responses are low in HIV-1 infected individuals with CD4 counts of less than 400/mm3 if the in vitro assay system is constructed to best mimic the in vivo situation. These results suggest that the lack of functional antibody responses to HIV may play an important role in viral pathogenesis.