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R e a c t i v e lysls is the consequence of the interaction of two reagents. One, previously called indicator, is a normal s e r u m protein and has been identified as C7 (1); the other is generated b y c o m p l e m e n t activation from certain sera (referred to as " r e a c t o r " sera) and has been called a c t i v a t e d reactor. I n this paper the n a t u r e of the activated reactor is explored and the course of the reaction analyzed. Materials and MethodsThese are described in the preceding paper (1). Diisopropyl fluorophosphate (DFP) was obtained from Boots Pure Drug Co., Nottingham, England.1:10 phenanthroline hydrate was obtained from Hopkins & Williams, Ltd., Chadwell Heath, Essex, England. A 0.5 ~ solution was made in dimethylformamide and this was diluted 1 : 50 in the E/EDTA/agarose mixture. 1Quantitation of Activated Reactor.--An estimate of activated reactor was obtained by filling a standard size well in an E/EDTA/agarose plate with activated reactor and allowing diffusion to occur for 18 hr at room temperature. The plate was then immersed in 1: 50 guinea pig EDTA complement and incubated at 37°C for 6 hr. The diameter of the zone of lysis was measured. RESULTS Fractionation of Activated ReactorAbsorption with Zymosan (4 mg/ml) and Precipitation as Euglobulin.--These were performed in the usual way. Reactor activity in the euglobulin fraction, when redissolved in 0.3 N NaCI, pH 7.2, was stable on storage both at 4°C and at -20°C. Addition of glycerol to a final concentration of 25% allows the activated reactor euglobulin to be stored, certainly for some months, at --20°C without loss of activity.

This paper describes the characteristics of the indicator factor (I) which takes part in reactive hemolysis and its identification as the seventh component of complement. I was shown to be a beta globulin with a sediment coefficient of 5.7S and a molecular weight of about 140,000. Experiments on the depletion of I activity with anti-I antiserum or with activated R euglobulin showed that I was a late acting complement component necessary for the lysis of cells after the EAC142 stage. Complement component analysis of purified I fractions excluded all known components except C7. The physicochemical characteristics of I are compatible with published data on C7. The method of quantitation described represents a convenient method of testing for C7.