Yeastolate is effective in promoting growth of insect cell and enhancing production of recombinant protein, thus it is a key component in formulating serum-free medium for insect cell culture. However, yeastolate is a complex mixture and identification of the constituents responsible for cell growth promotion has not yet been achieved. This study used sequential ethanol precipitation to fractionate yeastolate ultrafiltrate (YUF) into six fractions (F1-F6). Fractions were characterized and evaluated for their growth promoting activities. Fraction F1 was obtained by 65% ethanol precipitation. When supplemented to IPL-41 medium at a concentration of 1 g L(-1), fraction F1 showed 71% Sf-9 cell growth improvement and 22% beta-galactosidase production enhancement over YUF (at 1 g L(-1 )in IPL-41 medium). However, the superiority of F1 over YUF on promoting cell growth gradually diminished as its concentration in IPL-41 medium increased. At 4 g L(-1), the relative activity of F1 was 93% whereas YUF was 100% at the same concentration. At 1 g L(-1), four other fractions (F2-F5) precipitated with higher ethanol concentrations and F6, the final supernatant, showed growth promoting activities ranging from 32 to 80% as compared to YUF (100%). Interestingly, a synergistic effect on promoting cell growth was observed when F6 was supplemented in IPL-41 medium in presence of high concentrations of F1 (>3 g L(-1)). The results suggest that ethanol precipitation was a practical method to fractionate growth-promoting components from YUF, but more than one components contributed to the optimum growth of Sf-9 cells. Further fractionation, isolation and identification of individual active components would be needed to better understand the role of these components on the cell metabolism.