This paper presents an improved method of pricing vulnerable Black-Sch
oles options under assumptions which are appropriate in many business
situations. An analytic pricing formula is derived which allows not on
ly for correlation between the option's underlying asset and the credi
t risk of the counterparty, but also for the option writer to have oth
er liabilities. Further, the proportion of nominal claims paid out in
default is endogenous to the model and is based on the terminal value
of the assets of the counterparty and the amount of other equally rank
ing claims. Numerical examples compare the results of this model with
those of other pricing formulas based on alternative assumptions, and
illustrate how the model can be calibrated using market data.