Three mouse cyclin-like (CYL) genes were lsolated, two of which are regulated by colony-stimulating factor 1 (CSF-1) during the G1 phase of the macrophage cell cycle. CSF-1 deprivation during G1 leads to rapid degradation of CYL proteins (p36^CYL) and correlates with fallure to initiate DNA synthesis. However, after entering S phase, macrophages no longer require CSF-1 and can complete cell division without expressing CYL genes. During G1, p36^CYL is phosphorylated and associates with a polypeptide antigenically related to p34^cdc2. The timing of p36^CYL expression, its rapid turnover in the absence of CSF-1, and its phosphorylation and transient binding to a cdc2-related polypeptide suggest that CYL genes may function during S phase commitment.