BACKGROUND: Safety considerations require that biological products for human use are free from any agent that might pose a potential health hazard. One method to detect the presence of retroviral particles is the reverse transcriptase (RT) assay. This assay is capable of detecting all infectious retrovirus particles, irrespective of genome or protein composition. Recently, a family of ultrasensitive RT tests, named product-enhanced reverse transcriptase (PERT) assays, has been designed with a detection limit that is 10(6) - 10(7) times lower than that of conventional RT tests. OBJECTIVES: To investigate with the PERT assay whether RT activity is detectable in live attenuated virus vaccines and to characterize eventual RT activities. STUDY DESIGN: A total of 12 different monovalent and one trivalent virus vaccines containing live attenuated viruses were tested for RT activity with the PERT assay and a conventional RT test. RT activities were investigated with respect to their susceptibility to RT inhibitors, association with physical particles, and their possible origin. RESULTS: One trivalent and five different monovalent vaccines contained RT activity when tested with the PERT assay, but were negative in a conventional RT assay. All lots tested of these vaccines showed RT activity. The activity in all vaccines was sensitive to AZT-triphosphate and ddTTP and at least part of it was associated with particles. Mg(2+)-dependent RT activity banded at a density of 1.14 g/ml. All positive vaccines were produced using chicken cells. CONCLUSIONS: The data indicate the systematic presence of partially particle-associated retroviral reverse transcriptase in attenuated live virus vaccines that are produced in chicken-derived cells. The identification and further characterization of these particles, as well as the elucidation of possible interactions with the human organism are imperative goals despite the fact that these vaccines have been safely used for many years.