MANY previous investigations on antibody production in tissue culture have utilized indirect systems for antibody detection1,2. Others have examined the incorporation of isotope-labelled amino-acids into antibody by tissue-cultured cells3. There are few reports concerning the production of precipitating antibodies to protein antigens by cells in tissue culture for more than 1–2 days, and these are often unconvincing. Preliminary experiments in our laboratory with fluid media from cultures of immune lymphoid tissue fragments were not successful in demonstrating precipitins. However, it was thought that the combination of the sensitivity of the agar-gel diffusion technique with the continuous antibody production of cells in tissue culture possessed possibilities of demonstrating precipitating antibody production. Accordingly, the following techniques were developed to reveal anti-bovine serum albumin antibody production by immunized lymph node cells in tissue culture.