We prospectively studied the role of MP in childhood LTRI. Children aged 3 months to 12 years with a chest X-ray infitrate were included. Excluded were immunodeficiency disorders, aspiration, congenital malformations, nasogastric tube feeding, cystic fibrosis and severe retardation. A complement fixation test and an IF assay measuring IgM and IgG antibodies on day I and 14 were performed to confirm infection with MP. A four-fold rise in IgG titers or a positive IgM titer were considered diagnostic. On day 1 a nasopharyngeal aspirate was taken for culture of MP and PCR analysis. The primers used in PCR analysis were: Myc 16s and Myc PI. Over a period of 11 months 34 children (ISF, 19M) with a mean age of 5.5 years were included. In 10 patients (29%) MP infection was detected. The results of serologic testing(SER.). culture(CULT.