AbstractAliquots of 213 specimens were assayed for C3, C4 and CH50. A graphical presentation of the data was obtained by a block diagram. This showed a generally flat plane with good agreement to quoted normal ranges.Only 2 of 68 specimens with C3 and C4 normal have a low CH50. If either the C3 or C4 is low then the CH50 may be normal or even high, but if both fractions are low then so is the CH50.A low CH50 can occur with both C3 and C4 in the normal range or even higher. This implicates other complement factors as being deficient with a sufficiently high occurrence rate to require that the CH50 be kept as a routine laboratory procedure.