The modified algorithm for laboratory confirmation and differential diagnosis of measles infection was developed and used in the laboratory studies of the Russian Laboratory Network (RLN) on Measles/Rubella Surveillance within the routine and active measles infection control. The algorithm consists in detecting the IgM, IgG and IgG avidity measles virus antibodies. To approve the modified algorithm sera samples of 637 patients with the measles diagnosis and 423 patients with rush and fever were studied. The IgG avidity measles testing is advisable in the following cases: a) only one serum sample is available, b) the recommended time interval between the 1st and 2nd sera samples taking is not observed, c) no diagnostic increase in titers of the IgG measles virus antibodies is evidenced, d) for determination of the type of immune response (primary or secondary) and e) to exclude the "false positive" results at the stage of detecting the measles virus IgM antibodies. Moreover the data obtained evidenced the involvement of the nonvaccinated (79.7%) as well as of vaccinated with 1 or 2 doses of measles vaccine (20.3%) population into the measles epidemical process.