The counterflow pipeline architecture consists of two interacting pipelines in which data items flow in opposite directions. Interactions occur between two items when they meet in a stage. We present the design decisions for, and test measurements from, an asynchronous chip that explores the basic ideas of such an architecture. We built the chip in order to confirm proper operation of the arbiters required to ensure that each and every item flowing in one direction interacts with each and every item flowing in the other direction. Our chip, named "Zeke," was built in 0.6 /spl mu/m CMOS through the MOSIS fabrication facility. The maximum total throughput of the chip, which is the sum of the throughputs of the two pipelines, varies between 491 MDI/s (mega data items per second) and 699 MDI/s, depending on the amount of interaction that takes place. Under average data and operating conditions the performance of our chip was roughly halfway between these throughput values.