Abstract
Sperm motility in Limulus is initiated by a sperm motility initiating factor (SMI) that emanates from Limulus eggs. This report describes the partial purification of SMI (greater than 230‐fold purification with respect to protein content) with 40% recovery. SMI appears to be a hydrophobic peptide of 500–2,000 MW. Although probably not purified to homogeneity, SMI is estimated to be active at a concentration of less than 0.2 μM.