The complement system is composed of approximately 20 plasma proteins and constitutes a major humoral defense and clearance system in the bloodstream. New complement deficiencies, i.e. C8α-γ deficiency (C8α-γD) and C3 hypocomplementemia (C3-hypo), were identified in rabbits. The C8α-γD and C3-hypo were transmitted as a simple autosomal recessive and condominant trait, respectively. The physiological characteristics and molecular bases for C8α-γD and C3-hypo were identified as follows: C8α-γD is characterized by dwarfism (non-pituitary), small litter size, small thymus, a low survival rate, severely reduced serum bactericidal activity and enhanced delayed-type hypersensitivity (DTH), and normal expression of α and γ genes, but abnormal co-translational processing of C8α gene (a mutation of the exon/intron junction of the C8α gene). C3-hypo is characterized by approximately 10% of normal serum C3 levels, a low survival rate, reduced serum bactericidal activity, suppressed DTH, and low levels of liver C3 mRNA (pretranslational defect), and C3 phenotypes are dependent on RFLPs of the C3 gene.