Measuring the sources and rates of predation on planktonic protists is of interest from a number of perspectives. In studies of protist population dynamics, mortality rates as well as division rates need to be assessed. From a predator's perspective, the abundance (and specific composition of assemblages) of planktonic protists can markedly influence a predator's growth rate. From the point of view of ecosystem studies, the intensity and selectivity of predation on planktonic protists can alter the pathways of material and energy transfer in pelagic food webs. The present method was developed to measure predation on ciliate protitsts by planktonic suspension-feeding copepods. This prey-predator interaction is addressed because of the suspected importance of ciliates as a prey resource for planktonic metazoans and the likelihood that ciliates are significant internmediaries in the transfer of microbial loop production to larger suspension feeders. The need for new methods to study predation on planktonic ciliates arises because many ciliates are quite fragile and lack recognizable hard parts. Conventional methods for collecting, concentrating, incubating, and fixing planktonic organisms frequently result in lysis of these cells. This is particularly treu fro ciliates in the order Oligotrichida and those members of the Choreotrichida that lack a lorica. Despite the particular focus of the method outlined here, the principles of this immunochemical method should also be applicable to other protist-predator studies.