Antibody responses of commercial broiler chickens vaccinated for Newcastle disease were assayed by haemagglutination inhibition and enzyme-linked immunosorbent assay, and the results from the two methods compared. Three-hundred-and-nineteen chickens from 37 farms were assayed. The correlation coefficient was 0.85. Analysis of individual slopes of data from the 37 farms indicated that at least eight chickens from a farm must be sampled to achieve an acceptable degree of parallelism.