A rapid in vitro assay for cell mediated immuntiy in guinea pigs to soluble antigens has been developed. This assay is based on changes in the rate of protein synthesis that can be detected within 4 hr after the addition of specific antigen to sensitized lymphocytes. Antigen concentrations as low as 10-3 µg/ml provide effective stimulation of immunized cells. This stimulation of protein synthesis exhibits antigenic specificity, cannot be transferred by serum, is not present in animals immunized so as to produce antibody and not cellular immunity and exhibits a different time course of development after immunization than does antibody synthesis. This assay is thought to represent a rapid in vitro correlate of cellular immunity suitable for the study of soluble antigens.