Murine beta-galactoside-binding protein, a protein classified as a soluble lectin, is shown to be a cell growth-regulatory molecule and a cytostatic factor. The growth-inhibitory effect is not related to lectin properties, and competition assays indicate that the protein binds to specific cell surface receptors with high affinity. It exerts control in G0 and at G2, both as a regulator of cell replication and as a cytostatic factor.