Occult viral agents in cell cultures may include (i) known viruses present at a concentration too low for detection and (ii) unknown viruses. For maximal safety regarding adventitious agents, a strategy is proposed which involves cellular cloning of candidate cell substrates for vaccines. This will render occult viruses present in either all cells or none, thus greatly facilitating their detection. Known and unknown members of virus families known to establish latency are best detected by molecular methods based on broadly conserved sequences and low stringency conditions. The presence of retrovirus particles, known or unknown, in vaccine harvests can be excluded by the Product-Enhanced Reverse Transcriptase (PERT) assay which detects particle-associated reverse transcriptase with the sensitivity of nucleic acid amplification tests. Latent retroviruses can be activated by stimulation with halogenated pyrimidines and/or azacytidine. Endogenous proviruses with no release of particles are of no concern.