Specific pathogen free hens and males were experimentally infected with Mycoplasma gallisepticum. Eggs were then collected, and a part was incubated and set for hatching. Mycoplasma cultures were performed on infected adults and antibodies to MG were analysed by use of slide agglutination (SA) test and commercial ELISA tests on adults and chicks sera and on yolks from non incubated eggs. Both ELISA tests could detect antibodies in yolks from non incubated eggs laid three weeks after infection. SA and the three ELISA tests revealed positive sera in chicks hatched from eggs laid as soon as one week after infection.