A variant band in the Bf polymorphism has been found in the serum of a healthy women, her mother, her sister, and two brothers. By direct comparison the band was found to migrate faster than F but slower than F1. It is highly probable that the band represents a new allele at the Bf locus, Bf 0.55. The new allele was transmitted in this family together with the HLA haplotype A11, B27.