Malnutrition produces a pronounced effect on the complement system of man and animals. There is little information, however, to indicate whether individual complement components are affected at different stages of age. In the following studies, we have demonstrated that in guinea pigs and in both conventional and germ-free rats, protein malnutrition adversely affects the complement system. The levels of complement components C2 and C3 are suppressed by protein malnutrition at an early stage while C4 and C8 are less affected by malnutrition. Experiments in germ-free animals demonstrated that decreased complement component levels can be directly associated with decreased synthesis and/or secretion of components. In conventional animals, further reductions in complement levels could occur as a result of infection associated with malnutrition.