The cell concentration and possible biological activities of the pneumococcal Forssman (F) antigen (membrane lipoteichoic acid) were examined in a number of physiological situations. In test tube cultures of pneumococci the concentration of the Forssman antigen per bacterium showed no significant fluctuations within a typical culture cycle. Purified F antigen had no effect on the activation of pneumococci to competence for genetic transformation, DNA mediated genetic transformation or adsorption of the pneumococcal phage Dp-1 to bacteria. Pneumococci grown in the presence of different amino alcohols (ethanolamine, N-monomethylethanolamine, or choline) exhibit differences with regard to both their ability to stimulate heterophile (haemolytic) antibody production in rabbits and in their ability to bind such antibodies. Choline-grown bacteria seem to cross-react with sheep red blood cells better than do the analogue-grown bacteria.