We plan to study three aspects of the paramecium surface protein system: mutual exclusion of serotypes, reversible transformation of types, and variation in clonal stability: 1. Cells of different serotypes can be shown to apparently undergo transformation when mixed if one serotype is first treated with 5-fluorouracil. We will carry out experiments to determine whether differential survival is occurring or whether cell interaction induces transformation. We will also attempt to reproduce these results using filters to separate the interacting clones in equilibrium dialysis chambers. 2. A cell-free fraction (the sediment of a brei appearing after centrifugation at 500g)has been shown to induce specific transformation. We plan to purify and characterize the factor(s) responsible and to determine how transformation is elicited. 3. Antisera prepared against chemically altered surface proteins and against various cell fractions, although devoid of immobilizing antibodies, induce specific transformations. By comparing antisera with similar transforming patterns in immunoelectrophoresis using homogenized cells, we hope to begin to characterize those factors in cells (apart from the surface proteins themselves) involved in serotype regulation.