Insulin is the most commonly used growth factor for sustaining cell growth and viability in serum-free Chinese hamster ovary (CHO) cell cultures. In the present study insulin and IGF-1 analogue (LongR(3)) were compared for their ability to support growth, viability, and production of two serum-free CHO cell Lines expressing recombinant protein. The first cell line, VA12, expresses protein B, and the second cell line, CL23, expresses protein C. Both molecules are recombinant cytokine receptors. VA12 will grow in serum-free media lacking growth factor, while CL23 requires either insulin or LongR(3) for cell growth. Both cell lines, however, require a growth factor for optimal performance under production conditions. In this study, LongR(3) was better able to sustain the viability of both cell lines under production conditions than insulin. These data indicate that while insulin and LongR(3) can both serve as growth and viability factors for CHO cells, LongR(3) is the preferred growth factor for cell lines VA12 and CL23.