Direct detection of respiratory syncytial (RS) virus antigen in nasopharyngeal secretions (NPS) provides the most rapid diagnostic test for RS infection, but more sensitive methods might be more beneficial in the study of virus shedding. RS virus RNA was extracted from cells stored at -70 degrees C either in suspension with added RNAse inhibitor or as a pellet without inhibitor. The RNA was reverse transcribed, the resultant cDNA amplified by the polymerase chain reaction and detected by ethidium bromide staining after electrophoresis through agarose gel (RT-PCR). Of 217 specimens tested, 106 were positive by antigen detection, 99 by RT-PCR, and 92 by virus isolation. In a series of 97 sequential NPS specimens from 15 infants in whom RS virus induced bronchiolitis was confirmed, antigen detection proved most sensitive in the first week after onset and RT-PCR detected most positive specimens in the second week. Storage of the cells as a pellet proved more satisfactory than storage as a suspension. A further round of amplification using nested primers increased the number of positive results obtained by RT-PCR. The sensitivity of antigen detection using directly labelled monoclonal antibody to RS virus was slightly higher than that of RT-PCR, but the specificity was slightly lower.