The effect of the compound N,N'-bis/methylisatin-beta-thiosemicarbazone/-2-methylpiperazine (bis-MIBTP) on immune response in BALB/c and Swiss mice have been studied in the course of vaccinia virus infection. Humoral response tested by neutralization and hemagglutination inhibiting antibodies was similar in compound-treated mice to this of untreated mice. Cell-mediated immune response, examined by spleen lymphocytes migration inhibition test, has been delayed or temporally depressed in bis-MIBTP treated mice as compared with the control group. High protective activity of the compound in vaccinia infected mice in spite of impairment of cellular immunity may indicate that antibodies have played an important role in recovery process from vaccinia infection.