An international collaborative study was performed to investigate the reproducibility of influenza serological techniques. Participants in seven laboratories representing five countries measured antibody to A/Sichuan/2/87 (H3N2), A/Taiwan/1/86 (H1N1) and B/Beijing/1/87 influenza viruses in 11 human sera and three postinfection ferret sera. Two different serological techniques were used, haemagglutination inhibition (HI) and single-radial haemolysis (SRH) and, although each technique was reproducible within laboratories, variability between laboratories was higher for HI (maximum variability 32-fold; geometric coefficient of variation, GCV, 112%) than for SRH (maximum variability 3.8-fold; GCV 57%). The use of a standard serum allowed direct comparison of HI and SRH data and, for each technique, a standard serum improved inter-laboratory agreement. For influenza A viruses there was a correlation between HI and SRH antibodies (correlation coefficient approximately 0.9). An HI titre of 1:40 in human sera corresponded to an SRH titre of 19-33 mm2. The results of the study indicate that two sera would be expected to contain different antibody levels if their HI titres differed by > fourfold and SRH areas differed by > 50%. Both SRH and HI possessed equivalent sensitivity for measurement of antibody to influenza A viruses but SRH was more sensitive for detection of antibody to influenza B viruses. The study provided valuable information about standardization of antibody assays.