A~tract. Regenerable maize calli of two inbred lines were exposed to 0 to 100 Gy of gamma rays or treated with 0 to 30 mM of N-ethyl-N-nitrosourea (ENU) to determine their effect on growth and plant regeneration capability. Both growth and plant regeneration capacity decreased with increasing levels of either gamma radiation or ENU; however, plant regeneration capacity was more sensitive to either agent than growth. The 50% inhibition dose (I50) for callus growth (fresh-weight gain) was approximately 100Gy of gamma radiation and 30 mM ENU. The 150 for plant regeneration capacity of treated callus was approximately 25 Gy of gamma radiation and 2.5 mM ENU. The decrease in plant regeneration capacity correlated with a change in tissue composition of the treated callus from a hard, yellow and opaque tissue to a soft, grayish-yellow and translucent tissue. This change was quantified by measuring the reduction of MnO~-to MnO2 (PR assay) by the callus. These results suggest that the effect of gamma radiation or ENU on plant regeneration capacity must be taken into consideration if these potentially mutagenic agents are to be used on maize callus cultures, for the purpose of producing useful mutations at a whole plant level. The data also suggest that the PR assay may be useful for predicting the actual plant regeneration capacity of maize callus.