Filtrates of trypsin-treated suspensions of Shigella paradysenteriae (ST filtrates) were dialyzed in closed systems. The dialysates thus obtained were often found to have complement-fixing activity against anti-whole-Shigella serum. Evidence of antigenic material in such dialysates was also given by other serologic tests and by the appearance of agglutinins to Shigella in sera of rabbits injected with the dialysates. When lymph node cells of normal rabbits were incubated in vitro with such dialysates, washed and then transferred to fresh irradiated rabbits, agglutinins to Shigella appeared in the sera of the recipients. Within given ranges, higher agglutinin titers were found in sera of recipients of lymph node cells which had been incubated with dialysates of higher complement-fixation titers or with higher concentrations of given dialysates. Antigenic fractions of pyridine extracts of Shigella, prepared essentially according to Goebel et al. gave similar serologic reactions with anti-Shigella serum. Such extracts were also found to be effective for in vitro incubation with lymph node cells. Results of cross absorption tests among dialyzed ST, the ST dialysate described here and the pyridine-extracted antigen suggested that these were antigenically similar. On maintaining the pyridine-extracted acetone-precipitated antigen in solution, there was evidence of continued dissociation from it of dialyzable, antigenically active material. This dialyzable material was also found to be effective for in vitro incubation with lymph node cells in cell-transfer experiments.