Summary: A spectrofluorometric method for the determination of histamine liberated from monkey lung tissue passively sensitized with human atopic sera is described. The close correlation of histamine liberated from sensitized tissue with P-K titer, the loss of activity by heating sera at 56°C for 1 hr, and the failure of both rabbit and rhesus monkey precipitating antibody or human blocking antibody to release histamine indicate that this method provides an in vitro estimation of the reagin content of the sera from allergic patients. The sensitivity of the method described may also permit the detection of reagin in the sera of allergic patients not detectable by the P-K test. It is suggested that the use of adult monkey lung tissue may provide a means to study the mechanism of antibody fixation.