The serum of a 10-year-old boy was found to be totally deficient in hemolytic C1 activity. The lack of C1 activity was due to a selective deficiency of C1q whereas the titres of the other components including C1r and C1s were found within the normal range. We looked for C1q functionally as well as immunochemically. There was no decrease of serum immunoglobulins IgG, IgM, or IgA. Addition of highly purified human C1q to the serum restored the whole complement activity as well as the C1 activity. In ultracentrifugation studies the sedimentation rates of the restored C1 molecules were found mostly in the 18.6S region when the experiment was performed with 0.06 µ VBS containing gradients; however, C1 activity was found only in the 12.6S region when a gradient with 0.15 µ VBS was used. The dose-response curve of the restored C1 molecule showed the same nonlinear shape as that of C1 in normal human serum.