A survey of antigenic types, as revealed by immobilization reactions, among 78 strains of variety 1 of T. pyriformis grown in monoxenic cultures at 25°C discloses a minimum of six distinct classes. These are called the H serotypes, and are designated as types Ha, Hb, Hc, Hd, He and Hf. The terminal members of highly inbred series (Families A, A1, B, D, etc.) are of a single immunological class, but different inbred series show different serotypes. Early representatives of the inbred series show in contrast considerable serotypic variation. The distribution of serotypes is generally consistent with the genetic hypothesis developed in a parallel study, but new serotypes arise occasionally through some mechanism not yet adequately studied.