The addition in vitro of DNP-PLL or DNP-GL to lymph node cell cultures derived from genetic responder animals immunized with these materials led to a significant stimulation of ³H-thymidine incorporation into DNA. However, the addition of DNP-PLL or DNP-GL to lymph node cell cultures from nonresponder animals immunized with these materials failed to produce any stimulation of DNA synthesis. Furthermore, the addition of DNP-PLL to lymph node cell cultures from nonresponder animals immunized with DNP-PLL·BSA or DNP-PLL·OVA also failed to stimulate cell proliferation in spite of the fact that the lymph node cells of these animals were producing anti-DNP-PLL antibodies.

The above facts suggest that the function of the PLL gene product is to act at an early crucial step in the immune mechanism to form an antigen-inducer complex. The specificity of this early step may be of a simple order and different than that of the antibody which is later produced in the immune response.