u03bcm 4 6 u00d7 250 mm using a step gradient of 0 5 min 5 5 min 10 25 min 15 30 min and 40 15 min 1 propanol in 0 5 m formic acid 0 4 m pyridine ph 4 0 with a flow rate of 35 ml hr aliquots 20 u03bcl of fractions 3 min were assayed with the radioreceptor assay peak v g 100 was composed of several peptides which were equally active with or without trypsin digestion chromatography on rp 18 revealed opioid activity in the calibrated met and leu enkephalin positions and also in two regions before the elution position of met enkephalin in fractions 6 and 7 and one region after the elution position of leu enkephalin fraction 16 a similar chromatographic pattern was obtained from bovine striatal extracts although the relative amounts of these additional peptides differed the peptide eluting in the leu enkephal in position was purified to homogeneity by further chromatography on rp 18 it was shown to have the amino acid composition tyr 1 gly 2 phe 1 and leu 1 this represents the first chemical characterization of pure leu enkephalin from a natural source in order to determine whether fractions 6 7 and 16 were indeed separate entities differing chromatographically from the enkephalins these fractions were lyophilized redissolved and rechromatographed each opioid peptide eluted at its original position these three fractions 6 7 and 16 were characterized by comparing their activity in the radioreceptor assay and the n and c terminal directed enkephalin radioimmunoassays as shown in table ii all the fractions interacted well with the n terminal specific antiserum none of the three fractions interacted very well with the c.