The infectivity of Trichinella spiralis L1 larvae was examined in Swiss CD-1 mice after their maintenance in conventional cell culture media under different atmospheric conditions. Larvae isolated from the infected mouse carcasses were cultured for 24 hr in Roswell Park Memorial Institute (RPMI) medium, minimum essential medium (MEM), 199 medium, and Hank's balanced salt solution (HBSS) medium under anaerobic, microaerobic, and 5% CO₂ conditions. Only those larvae maintained under anaerobiosis in all media retained their infectivity in mice. The larvae maintained microaerobically and under 5% CO₂ lost more of their infectivity when cultured in RPMI medium and MEM (>96%) than in 199 and HBSS (>78%).