The cell line Pr8/22 has been derived from P388, a methylcholanthrene-induced lymphoid neoplasm originating in DBA/2 mouse and converted to ascitic form in the first mouse transfer. Pr8/22 is resistant to daunorubicin and is multi drug resistant (MDR). Cells should be treated with the drug at least once a month and before freezing, however after resuscitating daunorubicin should not be added until the first passage. It is advisable to keep a backup culture which is not drug-challenged.