With the use of conglutination assay and different anticomplementary agents, the lytic intermediate formed during interaction of sensitized sheep erythrocytes and diluted bovine serum was shown indirectly to be SHEACBo142. Lysis of this cellular intermediate by guinea pig C3-9 was completed in about 6 min of incubation at 37°C at ionic strengths between 0.075 and 0.125. In contrast to the lability of other corresponding intermediates, SHEACBo142 was extremely stable in low ionic strength buffer (GGBS++) with divalent cations: up to 18 hr at 37°C and 6 days at 2°C. Change of the ionic strength of the buffer to iso-ionicity resulted in gradual decay of the lytic intermediate, whereas presence of EDTA induced a precipitous decay of SHEACBo-142, whose reactivity could be restored with bovine euglobulin or functionally pure human C1. The apparent paradox, that is, that bovine complement was non-hemolytic to sensitized sheep erythrocytes and inhibitory to the lytic activity of guinea pig terminal complement components on SHEACBo142 but hemolytic to sensitized rabbit erythrocytes, further emphasized that in vitvo hemolytic complement assays do not necessarily correlate with in vivo functional activity of complement-dependent or -mediated processes.