Two new procedures for the isolation of interstitial cell-stimulating hormone (ICSH) from sheep pituitary glands have been described in recent years1,2. Sedimentation coefficients (s20°,w) of the hormone were reported to be 2.47 in phosphate buffer1 of pH 6.8 and 2.32 S in glycine buffer2 of pH 6.0. From these values, together with values for the diffusion coefficient and partial specific volume, these authors1,2 computed the molecular weight of ICSH to be 28,000-30,000. We have recently re-examined the sedimentation behaviour of ovine ICSH and found that the molecular weight of the hormone changes with the pH of the buffer. Results also indicate that the ICSH molecule forms reversible aggregates. We wish to report herein the molecular weight of the ICSH monomer.