Abstract An assay is described for the rapid microtitration of simian adenovirus 7 (SA7) particles capable of inducing adenovirus CPE (rounded, swollen cells). Utilizing the assay for cytopathic particles, it was found that only one passage of standard SA7 at high m.o.i. results in virus stocks containing an excess of noninfectious cytopathic particles. These stocks also contain an excess of defective virions which retain the ability to enhance the replication of human adenovirus type 5 during mixed infection of monkey cells. SA7 stocks produced on two SV40-transformed simian cell lines contained larger fractions of these defective virions than stocks produced on normal simian cell lines. Autointerference activity was not observed in SA7 stocks containing an excess of defective cytopathic particles nor were there detectable levels of shortened viral genomes.