Lipopolysaccharide from E. coli (LPS) and purified protein derivative from tuberculin (PPD) increased the [3H]thymidine incorporation of chicken spleen cells in culture. No such stimulation was observed with dextran sulphate. Thymus and bursa lymphocytes were not stimulated by any of these compounds. Spleen cells from chickens chemically bursectomized with cyclophosphamide treatment showed decreased responses to LPS and PPD, but responded normally to the T-cell mitogen concanavalin A. None of the tested mitogens induced polyclonal antibody synthesis or directly enhanced the primary antibody response to sheep erythrocytes (SRBC) in spleen cell cultures. LPS-coated SRBC, however, enhanced the in vitro antibody response to SRBC. The results suggest a moderate proliferative response of chicken lymphocytes to LPS and PPD, possibly involving B cells, but not further effects comparable to those on mouse B lymphocytes.