An increasing amount of loss due to mortality and morbidity is being seen in the broiler industry, however if the chickens are bred to have increased disease resistance these losses may be reduced. The study examined the impact that breeding broilers to have a stronger immune response to Escherichia coli would have within a commercial broiler population. Comparisons were made between hens with high or low responses to E. coli vaccination, commercial broilers and crosses (high x commercial, low x commercial and high x low). The chicks were test under a standard vaccination program during 1997 and 1998. All incidences of mortality were recorded, while body weights and blood samples to determine the anti-body response of natural exposure to E. coli and vaccination with Newcastle disease virus was taken from a sample group within the pen, depending on group size. The commercial line had the highest growth rate, moderate growth rates were seen in the high x commercial and low x commercial line, and the slowest growth rates were observed in high, low and high x low broilers. The highest antibody response was seen in the high line and the lowest response can from the low line. Moderate antibody responses were seen in the crosses and the commercial line, the lowest response out of the three from the low x commercial line and the highest from the high x commercial line, with the commercial line between the two. There was a high strong correlation between the chickens’ antibody response to Newcastle disease and E. coli, which may indicate that there is a common genetic component in the immune response to these antigens. The commercial line had the highest mortality rate and the lowest mortality rates were seen in the slowest growing lines (high, low and high x low). The mortality rate of the low x commercial line was 33% higher than the high x commercial line. The results indicated that there might be an interaction between the antibody response and growth rate on the mortality due to infectious disease