It looks to me as though you need a model to represent the survivals.

I would suggest that it be something like:

where

represents the probability that a mouse rejects on day

,

is the probability the graft survives the trial and

is probaility that it rejects on day

given it rejects. Then assume given it rejects that

has a normal distribution (remember that the recorded day is rounded to the appropriate integer).

Now you have two things to test:

1.That the probability that the grafts survive are equal

2.That the mean times to graft rejection given rejection are equal.

RonL