The effect of a 4-day treatment with 0.5% sulfaguanidine mash beginning at the time of inoculation was primarily coccidiostatic. Within 4 days after termination of therapy the treated birds, heretofore alert and active, became dull, lost wt. and began discharging large numbers of oocysts. Four-day treatment, initiated 1 or 2 days after inoculation, resulted in a 4-day delay in oocyst discharge, a moderate reduction in oocyst output, a retardation in wt. gains. Treatment begun 3 days after inoculation prevented symptoms and greatly reduced oocyst output but did not prevent some retardation in wt. gains. Birds placed on treatment 4 or 5 days after inoculation exhibited the same symptoms as the inoculated controls. Birds reinocu-lated 18 days after cessation of treatment showed a marked resistance to reinfection.