Client SSL Certificate Selection

I am using SoapUI 3.5 to test external vendor web service that requires two-way SSL certification. I have set up JKS key store and the callout seems to operate OK. The SSL cert tab on the response side indicates properly the certificates involved in the handshake. The thing is however that there is many certificates in that keystore. I don't see any option to identify the client cert (its alias or something) I want to present during the callout...

So my  question is: how does SoapUI pick the client certificate to send over to the server?

Thanks

-Alesh-