The fraction 1 of the insulin mediator (M. W. = 3700-4000) partially purified from the supernatants which were extracted from the pig liver particulate fractions incubated with insulin, (D-ala)B0 insulin and DesB1DPI respectively showed the capability to stimulate lipogenesis in rat adipocytes. The lipogenesis-stimulating activities of the fraction 1 generated were in order of inducers, i.e. insulin greater than (D-ala)B0 insulin greater than DesB1DPI. Significant correlation appeared between the values of 230 nm absorbances and activities of the mediators which were generated from liver particulate fractions incubated with insulin and its analogs. The coefficient of correlation of the values of O. D.230nm to the lipogenesis stimulating activities was 0.74 (n = 14, P less than 0.01). The results also showed that the yields of the insulin mediator reflected the biological activities of the hormone and its analogs, suggesting that the mediator is possibly a second messenger of insulin. Both the 230 nm absorbance and lipogenesis-activity peaks of the mediator were shifted to the right and molecular weight of the mediator became smaller (about 2000) when it was treated by 50 mmol/L formic acid. But its value at 230 nm absorbance and activity remained unchanged. Therefore, the fraction 1 of the insulin mediator seems to be depolymerized or dissociated into a smaller molecule under the acidic condition. The mediator of fraction 1 was neuraminidase- and pronase-sensitive so that it may be a glycopeptide chemically in nature.