A one-to-one complex of a 45,000-mol-wt protein and actin was purified from unfertilized eggs of the sea urchin, Hemicentrotus pulcherrimus, by means of DNase l-Sepharose affinity and gel filtration column chromatographies. Effects of the complex on the polymerization of actin were studied by viscometry, spectrophotometry, and electron microscopy. The results are summarized as follows: (a) The ...