Helper factors specifically stimulating cooperative antibody responses by normal mouse spleen cells to a dinitrophenylated protein antigen from Streptococcus mutans (DNP-SA) were produced in vitro from monkey peripheral blood leucocytes and mouse spleen cells. The factors were partially characterized by gel filtration on Sephadex G-75, isoelectric focusing, treatment with heat and degradative enzymes and binding to specific immunoadsorbents. Gel filtration of both the monkey and mouse factors showed coelution with human serum albumin, suggesting a molecular weight of approximately 70,000. The isoelectric points fell within the range of 4.9-5.2 for monkey and 6.4-6.7 for the mouse helper factors. The glycoprotein nature of both factors was suggested by their lability to heat and sensitivity to pronase and neuraminidase. The factors carried a small fragment of the stimulating antigen and showed specific binding to SA but not to keyhole limpet haemocyanin (KLH). Monkey factor bound to rabbit antisera directed against the Fc portion of monkey IgM, but not to the IgG or IgA isotypes. The mouse factor contained determinants coded for by the I-Ak but not I-Jk subregion of the MHC. Both factors were absorbed by an antiserum to helper factor raised in rabbits against a KLH-specific mouse helper factor as immunogen. A corresponding antiserum to suppressor factor failed to adsorb either factor. This emphasizes the specific identities of helper and suppressor factors and suggests an evolutionary relationship between those derived from monkey and mouse leucocytes.