Data are presented to show that there are three serum fl-globulin types in laboratory mice controlled by a pair of alleles. Each allele appears to give rise to three electrophoretically distinct zones in starch gel. Within the inbred strain AI AGS there was variation between mice in the intensity of staining of the three zones. Reciprocal mating data gave no evidence of an effect of fl-globulin type on segregation ratios as has been reported for cattle.