In serum specimens from 50 normal adults, a high correlation was found between the sum of the levels of the control proteins of the C3b amplification loop, C3b inactivator and beta 1H, and the sum of the component proteins, C3 and factor B (R = 0.83), when the concentrations are expressed as percent of the normal adult level. The slope for this regression was 0.98 and the y intercept practically zero, indicating that under normal conditions, beta 1H + C3b inactivator = C3 + factor B +/- 35% (95% error of the prediction). Biological significance of the combined values was suggested by the fact that their equality was largely preserved in serum specimens from cord blood, from infants, and from patients with idiopathic nephrotic syndrome, malnutrition, chronic illness, and partial C3b inactivator deficiency even though, individually, control and component protein concentrations deviated from normal in many of these conditions and, as single proteins, did not correlate with each other as in normal subjects. On the other hand, C3 + factor B was disproportionately lower than beta 1H + C3b inactivator in subjects heterozygously deficient in C3 and in those with hypocomplementemic glomerulonephritis and was often disproportionately high in those with an acute-phase reaction. The data indicate that for meaningful interpretation of the serum C3 level, the concurrent levels of factor B, C3b inactivator, and beta 1H must be taken into consideration.