Chronic latent infection with Besnoitia jellisoni, an obligate intracellular protozoan, in golden hamsters is highly sensitive to corticoids. Fatal relapse from renewed microbial proliferation can be brought about as early as 6 days after initiation of corticoid therapy, presumably by abolishing acquired immunity. By an assay which uses as end point the dose producing relapse on the 12th day, potency ratios have been determined for 22 modified progestins and corticoids. Potentiation of anti-inflammatory effect and of glycogen deposition, brought about by 1,2-dehydrogenation, 6-methylation, 9-fluorination, 16-hydroxylation or 16-methylation of the steroid nucleus, is accompanied by a similar but not identical potentiation of the infection-enhancing activity.