Immune response in a six-year-old girl with a rare complement defect, namely selective complete C1q deficiency, was studied. Her cell-mediated immune response (delayed hypersensitivity skin tests, E, EAC rosettes, in vitro lymphocyte transformation with phytohemagglutinin), isohemagglutinin titers, serum immunoglobulin levels, antibody titers to tetanus, Epstein-Barr virus, keyhole limpet hemocyanin, pneumococcus and bacteriophage 0 x 174 were all normal. However, she had abnormal kinetics of the antibody response to bacteriophage following primary and secondary immunizations. Her antibody titers reached a peak four weeks after primary immunization and three weeks after secondary immunization, while the titers of controls peaked at two weeks and one week, respectively. In this study we could not prove the hypothesis that defective antibody response caused recurrent infections in this patient. An alternative hypothesis is the possible role of defective clearance of immune complexes in the development of severe infections. Patients with selective complete C1q deficiency form immune complexes with bacterial or viral antigens, thus activating the classical complement pathway. As a result, very low C1q levels decrease even further, leading to a severe immunodeficiency and recurrent infections. Future studies in this area may help to explain the mechanism(s) responsible for infections in this rare type of complement defect.