Human beta-endorphin-like immunoreactive substances (beta h-EI) in human cerebrospinal fluid (CSF) were determined radioimmunologically. The cross reactivity of the antibodies to human beta-endorphin (beta h-E) amounted to 40% for human beta-lipotropin (beta h-LPH) whilst it was less than 1% for leu- and metenkephalin, alpha- and gamma-endorphin, fraction I and II [5], substance P and alpha-MSH. Prior to radioimmunological determination, an adsorbtion of beta h-EI from CSF with silicic acid was carried out and followed by a desorbtion, using a mixture of aceton/hydrochloric acid. This method was chosen because of the ratio of beta h-LPH to beta h-E in the desorbate can be shifted in favour of beta h-E owing to the variation in recoveries r (r beta h-LPH = 33%, r beta h-E = 64%). On the one hand, this enables a more specific determination of beta h-E and, on the other hand, and separation of any peptidase than may be present [9]. An adsorbtion/desorbtion of 2 ml CSF surfaces to prove the presence of 20-150 pg/ml (65-48 fmol/ml) of beta h-EI. The CSF of 28 patients with various neurological diseases was examined and 24 of them had concentrations of 20-70 pg/ml beta h-EI. The remaining four which had concentrations less than 20 pg/ml, came from meningitis patients undergoing corticoid therapy. A purchasable RIA kid was tested for its determination of beta h-E and was found to be unsuitable.