Antigenic activity related to the amyloid serum protein SAA was observed in indirect immunofluorescence studies on human placental tissue. Positive staining with anti-SAA antisera was localized to the cytoplasm of cells scattered within the mesenchymal stroma, thought to be fibroblasts, and to foetal stem vessel endothelium and some individual fibrillar structures in villous stroma and perivascular tissue. This immunofluorescent staining was specifically inhibited by protein SAA. In contrast, no immunofluorescent staining was achieved using antisera to the amyloid protein AA. Absorption and immunodiffusion studies have further suggested that anti-SAA antisera may recognize in human placentae only a very limited number of the antigenic determinants present in protein SAA but not in the smaller protein AA. The results support previous observations that protein SAA-like antigenic material can be found in normal human tissue.