Eggs from hens infected with Mycoplasma gallisepticum and hens free from M. gallisepticum and M. synoviae were hatched and samples of blood were taken between 1 to 21 days after hatching. Antibodies (M. gallisepticum) responsible for both rapid serum agglutination (RSA) and haemagglutination inhibition were detected in the sera of progeny of infected hens, but these declined until they were undetectable by 18 days of age. By 4 days after hatching only approximately 65% of the birds were positive to the RSA test. This work re-emphasises the need to sample chicks early in life when attempting to detect maternally derived antibody.