The growth inhibitory effects of combinations of T-2 toxin and verrucarin A on the yeast Kluyveromyces marxianus was studied. A combination index value was derived to indicate the type of interaction that existed between the binary mixture of these two toxins at various ratios and the target yeast cells. The type of interaction was dependent on the ratio of the toxins used to attain a particular level of growth inhibition. Further, the least change in the type and intensity of interaction or the maximally quiescent ratio (MQR) was found to be unique to the growth medium. In a rich medium, the MQR was 1.0 microgram/ml T-2 toxin:0.75 microgram/ml verrucarin A, where the two toxins had a very stable synergistic interaction over a 2 or 3 log value concentration range. Decreasing the nutrients changed the MQR to 1.0 micrograms/ml T-2 toxin:0.38 microgram/ml verrucarin A. Halving the concentration of the cells in the assay changed the MQR to 1.0 microgram/ml T-2 toxin:6.0 micrograms/ml verrucarin A. We have previously shown that the hierarchy of trichothecene toxicity in yeast bioassay is verrucarin A > roridin A > T-2 toxin > diacetoxyscirpenol > HT-2 toxin. The MQR of these toxins in combination with T-2 toxin follows the same order. This study shows an exception to the above order in that verrucarin A and roridin A exchange places.