Virus subpopulations with variable virulence, immunogenicity, and infectivity to pigs were readily generated by passaging Tengani isolate of African swine fever virus, either biologically cloned or uncloned, in Vero cell cultures. Avirulent virus populations which account for more than 99% of virus in an uncloned preparation of the 27th padsage are laboratory artefacts, perhaps do not exist in nature. Furthermore, attenuation of virulence did not occur uniformly in all subpopulations newly generated, and a continuous modulation of virus populations differing in immunogenicity and virulence took place in the same individuals inoculated with the 27th passage virus. The same virus preparation, appearing to be slightly virulent in pigs, contained at least a virulent subpopulation that was manifested only by further inoculating susceptible pigs with viremic blood collected at various times during the clinical course. A cloned virus after 23 passages in cell cultures generated a subpopulation (99.9%) which induced subclinical infection in pigs; however, the infection did not confer a solid immunity to homologous challenge with Tengani isolate in these pigs. The Tengani isolate contained subpopulations of virus with immunogenicities shared by the Lisbon '60 isolate and also contained at least one subpopulation specific for the Tengani only.