The cross reactivity of two currently used substance P (Sp) antisera (K 16 and K 25) with various analogues of SP, partial sequences of SP and other naturally occurring peptides were tested in a radioimmunoassay system. Both antisera were found to be C‐terminal directed, this tendency being most pronounced for antiserum K16. The capacity of antiserum K 25 to bind various SP‐related peptides correlated to the previously described biological activity of these peptides on the isolated guinea pig ileum. The results thus indicate that this antiserum binds to a biologically active part of the SP‐mol