The conditions for the production of histamine releasing factor (HRF) by guinea-pig spleen cells, thymocytes, and PBMC were investigated. Cultures of lymphoid cells were studied either alone or in the presence of mitogens or specific antigen (Ag) and the dialyzed cell free supernatant was tested for histamine releasing activity on English-short-hair-guinea-pig lung mast cells and blood basophils. Significant amounts of HRF were produced by guinea-pig spleen cells, thymocytes, and PBMC spontaneously while in animals immunized with Ag in CFA the production was enhanced on stimulation with phytohemagglutinin or specific Ag. Gel chromatography identified two distinct species of HRF with molecular weights of 50,000 to 70,000 and 5000 to 8000. The histamine release from lung mast cells as induced by HRF was temperature dependent and complete in 10 minutes at 37 degrees-C. An immediate ear swelling reaction was noted in guinea pigs following the intradermal injection of HRF. The most severe of these reactions was sustained over a period of 12 to 24 hours when higher doses of HRF were administered. These results suggest that guinea-pig lymphocytes can produce HRF that releases histamine from mast cell and basophils. HRF was active in- vivo. The intradermal injection of gel fractionated HRF induced an immediate ear swelling reaction. Gel fractionated HRF had no IL-1 activity. The guinea-pig mast cells released relatively small amounts of histamine, 8 to 20 percent, compared to the 30 percent of so released by the basophils. The authors conclude that the guinea- pig is a good model for various diseases.