To approach the genetic mechanism that turns off the synthesis of alpha-fetoprotein (AFP) after birth, we assumed that a change in this mechanism might affect the low basal level of AFP that can be detected in the adult organism. The concentration of AFP was therefore determined for serum from adult mice of 27 different inbred strains. With one exception, this basal level was between 34 and 173 ng/ml, which is about 10(5)-fold less than the serum concentration at birth. In one strain, BALB/c/J, the AFP level was found to be considerably increased; it was about 10-fold higher than in other strains at 9-10 wk of age. Two other substrains of BALF/c mice showed normally low AFP levels. Kinetic studies show that the rate with which AFP disappears from serum after birth is reduced in BALB/c/J mice as compared to other strains. The increased AFP level of BALB/c/J mice appears to be due to an increased rate of synthesis of AFP, since the rate of catabolism of AFP was found to be normal in these mice. Genetic analysis was performed by crossing BALB/c/J mice with mice having an ordinary AFP level, followed by determination of AFP levels in mice of the F1 and F2 generations as well as in back-cross mice. The results clearly indicate that the increased AFP level in BALB/c/J mice is controlled by a single recessive Mendelian gene, which has been named Raf (for regulation of alphafetoprotein). The Raf gene could be directly involved in the regulation of AFP synthesis, but it may also control AFP levels only indirectly, e.g., by regulating the synthesis of a hormone that controls AFP synthesis.