SYNOPSIS. Unabsorbed, complement‐inactivated antisera produced in rabbits were used in an immobilization system to study the serology of 31 strains of Tetrahymena pyriformis grown in axenic cultures at 26°C. Fourteen serological “groups” were found: 1. “H”: H, E, S, GC, L3, W, T, GHH, G1‐R and sublines W‐P and T‐P; 2. “PR”: PR; 3. “GL”: GL; 4. “LR”: LR; 5. “L1”: L1 and L2; 6. “GP”: GP and Ch‐S; 7. “WH”: WH6, WH14 and WH52; 8. “N”: N and HS; 9. “Y”: Y and TC; 10. “AA”: AA1, AA2 and AA4; 11. “F”: F and BF; 12. “Gf‐J”: Gf‐J; 13. “EZ”: EZ; 14. “Lava”: Lava. Not all of these “groups” were completely distinct; weak or variable cross‐reactions occurring for particular cultures and antisera during the course of several years of observation suggested a relationship in terms of serotype potentialities, for strains in groups 1 and 2, and in groups 2, 3, 4, 5, and 6. Non‐reciprocal cross‐reactions are tentatively explained in terms of population fluctuations of serotype within particular cultures; several other suggested explanations remain possible. On the whole, however, the strains remained remarkably stable in dominant serotype. Observations on Tetrahymena in paralyzing antisera, with respect to the extrusion of a gelatinous exudate, in general confirmed previous observations by others. “Chain” formation was noted upon recovery, under conditions suggesting that the chains do not always derive entirely from incomplete cytokinesis of dividing organisms but that at least sometimes an aggregation of separate individuals is also involved. Synchronized division was observed for recovering organisms. A gelatinous material was obtained by centrifugation of a large number of individuals; this material provided a degree of protection to the organisms against antiserum effects while other colloidal materials did not, but the protective effects proved nonspecific in the sense that they were not confined to particular strain‐antiserum combinations.

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SYNOPSIS. Studies cf 3 conjugating strains of Tetrahymena pyriformis grown in a bacterized medium indicated the presence of a system of alternative immobilizing surface antigens. Each strain was found to have the potential for the expression of 3 serologically unrelated antigens. These consisted of a “high temperature” antigen (expressed in the range of 20–35d̀C), a “low temperature” antigen (exhibited in cultures at 10d̀), and an antigen induced by growth in the presence of the above‐mentioned “high temperature” antiserum. Normally, by the immobilization reaction, only one of these antigens could be detected as present on any one organism at any given time. The “high temperature” antigens of two of the strains were serclogically related while that of the 3rd strain did not cross‐react with antisera to the other two. The “low temperature” antigens of the 3 strains were serologically related as were the antigens induced by growth in antiserum. Studies of inbred hybrids of two of the strains indicated a potential for still more alternative antigens as well as for the spontaneous expression of the antigen which in the parental strains could only be induced to appear by the presence of the specific antiserum. Study of the 3 strains in axenic medium indicated the existence of a more complex system and the possibility that two or more immobilizing antigens may be present simultaneously.