Eight strains of Streptococcus pneumoniae were tested in vitro for their ability to produce capsular antigen in the presence of penicillin. It was found that, provided 10⁶ to 10⁷ pneumococci/ml were present, capsular antigen could be detected during the 72 h in which the experiment was conducted, irrespective of whether penicillin was added at 0 h or 8 h, and even when no viable pneumococci remained. When fewer pneumococci were present, capsular antigen could not be detected at any time in the presence of penicillin. Control cultures, without penicillin, yielded detectable capsular antigen only when the threshold value of 10⁶–10⁷ pneumococci/ml was reached. It is concluded that the presence of penicillin does not influence the detection of pneumococcal capsular antigen, but demonstration of this antigen is totaly dependent on the number of pneumococci present.