Genetic selection for early humoral immune responsiveness against two simultaneous antigens, namely, heat-killed Escherichia coli and Newcastle disease virus vaccine (NDV), was performed in a heterogenic population of broiler chickens. The humoral immune response was measured prior to 24 days of age, depending on the vaccine. Chicks were divided into two populations according to their antibody response: a population of high responders to both antigens and a population of low responders. After four generations of selection, a significant difference was observed in the immune response of the two populations, and therefore, an attempt was made to identify possible differences in the segregation of major histocompatibility complex genes. Using alloantisera prepared in B-congenic White Leghorn, the high responders exhibited a high percentage of chickens having erythrocytes agglutinated with B5 antisera, and the low responders exhibited a high percentage of chicks with erythrocytes agglutinated with B15 and B19 antisera. The B13 antisera agglutinated cells of similar proportions of chickens in the two populations. A random commercial broiler flock was screened with the antisera, and correlation was high between B haplotype and antibody level to E. coli following vaccination. The present work indicated the possibility of a direct association between the genetic characteristics represented in the B complex and humoral antibody response in a broiler population.