Sera from obligate heterozygotes for deficiency of the C8 beta subunit of the eighth component of human complement (C8) were analyzed for the molecular composition of C8. The C8 alpha-gamma and C8 beta subunits were separated by SDS-PAGE, visualized by immunoblotting, and the resulting bands were quantitated by laser densitometry. The laser densitometric absorption data were set to 100 arbitrary units (AU) for both subunits of pooled normal human sera. The AU values of individual normal sera ranged from 45 to 150 AU for C8 alpha-gamma (median 99 AU) and from 45 to 140 AU for C8 beta (median 101), whereas the C8 alpha-gamma/C8 beta-ratio varied from 0.7 to 1.4. Sera from C8 beta-deficient heterozygotes differed, as expected, from the normal sera for the markedly reduced levels of C8 beta (20 to 90 AU, median 55 AU) and for the higher C8 alpha-gamma/C8 beta-ratio (1.3 to 3.5). High voltage agarose gel electrophoresis was used to separate free and C8 beta-bound C8 alpha-gamma. The migration of free and C8 beta-bound C8 alpha-gamma subunit was checked by hemolytic overlay gels and by second dimension SDS-PAGE and immunoblotting. Immunochemical evaluation of C8 alpha-gamma using this system revealed about 5-14% free C8 alpha-gamma in sera with normal C8 and higher levels, from 33-71%, in the C8 beta D heterozygous sera. Functional analysis confirmed the substantial increase of free C8 alpha-gamma in the heterozygous group. We conclude that the C8 in C8 beta D heterozygous sera is characterized by increased amounts of free C8 alpha-gamma due to reduced concentrations of the C8 beta subunit. This finding may help to identify individuals heterozygous for C8 beta deficiency.