The media and culture conditions required for in vitro stimulation of mouse lymphoid cells are described. The medium was arginine-rich and contained heat-inactivated human serum. A component of the human sera necessary for stimulation of the cells was a natural mouse cell agglutinin, which affected both background stimulation and the degree of induced stimulation with phytohemagglutinin (PHA). Absorption of the agglutinin from the human serum rendered the medium incapable of sustaining DNA synthesis in the presence of PHA.

The response to PHA of mouse spleen and thymus cells was age-dependent and, although this response was not present at birth, it rapidly rose to adult levels. Spleen cells from mice immunized with bacillus Calmette-Guérin (BCG) or sheep erythrocytes (SRBC) showed increased in vitro reactivity to added purified protein derivative (PPD) or SRBC stroma, dependent on the time of immunization.

The dose response curve for the SRBC stroma stimulated, immune spleen cells is compatible with a theory of cell to cell interaction being necessary for an in vitro reaction to antigen.

The possible role of the mouse cell agglutinin (AMLG) is discussed.

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