Intradermal injection of mice with ribonuclease antibody, followed by intravenous injection with ribonuclease, resulted in permeability increase, demonstrable by "blueing." The size of the blued area depends on the quantity of antibody injected and on the interval between the two injections. If antigen was injected first and antibody was injected subsequently, a similar increase in permeability was observed in animals having a complete complement system (MuB1-positive) and in animals which have a deficient complement system (MuB1-negative). Marked differences in response were observed between these two types of mice if antigen was injected some hours after the antibody. In MuB1-negative mice, a blueing reaction was not observed at intervals between injections (2½ hours if 3 µg N antibody and 15 hours if 25 µg N antibody were injected intradermally) at which MuB1-positive animals showed a marked permeability increase. At these intervals, blueing did occur in MuB1-negative animals if they were injected with the serum of MuB1-positive mice or with fresh guinea pig serum. Blueing was not induced if the serum of MuB1-negative mice or heated guinea pig serum was injected. The occurrence of two distinct phases of the cutaneous reaction, of which only one involves the complete hemolytic complement system, was deduced from these observations
