1. The participation times of a series of mixtures of horse serum and a pool of rabbit antihorse serum have been determined. The results have been presented in a table and isochrones drawn. Four distinct α procedure optima were found, suggesting that there may be at least four different antigen-antibody reactions concerned when horse serum reacts with a rabbit antihorse serum.
2. The reaction between horse serum crystalbumin and the same pooled rabbit antihorse serum has been investigated, and recorded-similarly.
3. Inhibition of precipitation was found to be more marked in the region of antigen excess than in the region of antiserum excess. Evidence is presented in support of the view that the occurrence of multiple zones may obscure this observation.
4. The results are compatible with the view that the second stage of the precipitation reaction is specific.
5. The isochrones correspond with Boyd's R type.