It is the episode where J. Peterman has an office party and Elaine ends up making out with some guy, who turns out to have a heroin habit. Mr. Peterman blamed himself because he claimed that he introduced him to a "chinaman" while on a business trip who could hook him up with heroin.
If that does not stir up your memory, it was also in the episode that Kramer and Newman tried to get the homeless to pull rigshaws.