Thirty-five synovial fluid (SF) specimens were examined for the presence of mast cells and for their histamine content. Mast cells were seen in SF cells from 27 of 35 fluids, and histamine was measurable in 19 of 34. There was a strong correlation between mast cell number and histamine content. No consistent relationship was found between either the mast cell number or histamine level and the patients' diagnoses, except that the 2 patients with systemic mastocytosis had markedly elevated values for both SF mast cell number and histamine content. SF mast cells from one of the mastocytosis patients were studied for histamine release; significant amounts of histamine were released upon exposure to anti-human IgE, but not compound 48/80. Thus, mast cells similar to those present in connective tissue are frequently present in SF in numbers which correlate with SF histamine levels. These mast cells contain active proteases and are capable of degranulation. Mast cells were consistently present in large numbers in the SF of patients with systemic mastocytosis, but their numbers were highly variable in fluids of patients with other diseases.