An experiment on sensitivity to spring frost in vine buds was undertaken in the champagne vineyard (Vitis vinifera L Chardonnay) during the springs of 1987-1989 by means of differential thermal analysis (first exotherm) performed in freezing chambers, providing either convective cooling (on vine directly in the field) or radiative cooling on twigs. The sensitivity of buds was analysed in terms of bud phase, surface wetness and ice nuclei bacteria (Pseudomonas syringae) concentration. Figure 1 shows the evolution with phenological phase of mean freezing point of dry buds for 3 yr independently of ice nuclei concentration. Figure 2 reports this freezing point for dry vine buds which contain < 103 bacteria, ie quasi-free of ice nuclei. Figure 3 reports for stage 9 of Eichhhorn and Lorenz the freezing point of dry buds versus ice nuclei concentration. The evolution of the mean freezing point at different stages is plotted on figure 4 versus mean ice nuclei bacterial concentration for both dry and wetted vine buds. From this figure, one can see that frost sensitivity of buds is increased not only by phenology but also by wetness of bud surface. On the contrary, the influence of ice nuclei bacteria is not so obvious. It seems that this bacterial influence could be different depending on the origin of the bacteria. In figure 5 at stage 7, the freezing point of contaminated buds (x) is higher than that of buds which contain the same amount of bacteria (Δ). The only minor effect of natural ice nuclei bacteria concentration on buds was observed on the percentage of surviving buds for a given temperature : the concentration of bacteria observed on killed buds (formula, see attached document) is a little higher in figure 6 than those in surviving buds (+).