Previous data showing that thiols can functionally replace macrophages in certain in vitro lymphocyte reactions have raised the possibility that macrophages generate SH groups in the medium. The SH activity of cell-free medium decreases considerably due to auto-oxidation. The presence of macrophages inhibited the spontaneous decrease of SH activity and even increased the number of free SH groups in the culture media. This was designated as SH generation. Resident as well as in vivo stimulated (NaIO4, thioglycollate medium, paraffin oil or BCG) macrophages have nearly the same capacity to generate SH groups when cultured in vitro. The SH-generating ability of macrophages depends on the viability and density of cells, and is greatly influenced by the serum concentration of the media. The majority of SH groups produced by macrophages could be demonstrated as albumin SH groups and proved to be more resistant to autooxidation than non-protein thiols. Moreover albumin could replace whole serum in respect of SH generation. It is suggested that macrophages by generating SH activity, may exert a non-specific helper function to lymphocytes or other cells in their environment.