FIGURE 1. Circulating antigens in the serum of a cynomolgus macaque infected with tetrathyridia in M. corti. Center well (c) contains RAS/E; wells 1 to 3 contain serum from host number 1 (splenectomized) at days 91, 98, 105 p.i.; wells 4 to 6 contain serum from host number 2 (intact) for the same time periods. yielded no precipitin bands for the same time period. No antigen could be detected in the serums of the control animals at any time. By use of the Ouchterlony technique, no antibody against tetrathyridia secretory/excretory antigens could be detected in the serums of the infected or control hosts at any time prior to the experiment. Perhaps host splenectomy, as demonstrated by Novak (1974, Int. J. Parasitol. 4: 165-168) in her work with mice and tetrathyridia, results in an increase in the parasitie burden. Splenectomy may have been responsible for the 500-fold increase in the parasite burden in host 1 over host 2 with a concomitant production of excess antigen. The cynomolgus macaque has been demonstrated to be susceptible to intraperitoneal infection by tetrathyridia of M. corti. In spite of the absence of clinical symptoms, heavy infections of this host by tetrathyridia may be demonstrated by the presence of circulating antigens. This study supported, in part, by a research grant (PCM 7718646) from the National Science Foundation.