This communication reports on the release of Mycoplasmavirus L1 after infection of Acholeplasma laidlawii with purified L3 virus. Release also occurred after transfection with certain restriction fragments from MV-L3 and MV-L1 genomes. Since circular molecules are efficiently taken up in polyethylene glycol-mediated transfection, inducing fragments were applied cloned in E. coli plasmids. Release was also observed after electroporation of cells incubated with MV-L1 replicative intermediate DNA and linear MV-L3 DNA isolated from virus particles, respectively. Released MV-L1 viruses were identified after virus plaque formation on indicator lawns according to plaque morphology and hybridization with labeled viral DNA probes as well as by DNA restriction analysis. Uninfected and untransfected cells from six laboratory strains of A. laidlawii (including a MV-L1 resistant one) were examined for the presence of MV-L1 DNA. They all bear MV-L1 DNA integrated in their genomes.