Two sandwich ELISA for the quantitation of C7 in human plasma--one based on a monoclonal (MAb), the other performed with a polyclonal anti-C7 IgG--were established. Results from ELISA as well as from isoelectric focussing and immunoblotting with both antibodies revealed that the MAb is directed to an allotypic epitope on C7. About 6.7% of the plasma samples did not react with this MAb. Since these findings cannot be explained by the known alleles, the existence of a new C7 allele, C7*9, was suggested, whose product C7-9 is not detected by the MAb. A gene frequency of 0.21 C7*9 was assumed. When using conventional techniques C7*9 is included in the frequency of C7*1.