Spermine at a concentration of 0.001 m initiated the reversion of penicillin-induced L-phase growth of Haemophilus influenzae to bacillary growth on penicillin L-phase medium. Reversion of L-phase colonies to bacillary colonies required 3 to 5 days. This spermine reversal of variant growth also occurred on penicillin induction medium in the presence of concentrations of tetracycline, erythromycin, and chloramphenicol that were not bactericidal for L-phase or bacillary inocula until 48 or more hr. Spermine was without protective effect against streptomycin and kanamycin, which were, in combination with penicillin, bactericidal at 24 hr. Spermine protection of L-phase variants against antibiotic toxicity was, therefore, related to initiation (by spermine) or bacillary growth from round bodies that survived for 24 or more hr at bacteriostatic levels of antibiotic.