First-generation schizonts of Eimeria callospermophili, E. larimerensis, and E. bilamellata from the Uinta ground squirrel, E. nieschulzi from the rat, and E. ninakohlyakimovae from the sheep were grown in cell lines of embryonic bovine intestine, kidney, and liver. Extracellular merozoites and mature first-generation schizonts with merozoites in monolayers were observed with phase-contrast microscopy for degree of motility. Various agents in minimal essential medium (MEM) were then added to the preparation; these included 0.2% trypsin, 4% bovine bile, 0.5% sodium taurocholate, 0.5% sodium glycotaurocholate, and 0.2% trypsin combined with 4% bovine bile or 0.5% sodium taurocholate; also, MEM with no agents added was used as a control. Little or no motility was observed in preparations to which MEM or trypsin was added. In all preparations to which bile, a bile salt, or one of these in combination with trypsin was added, the extracellular merozoites were markedly motile, and the merozoites still within schizonts except for those of E. ninakohlyakimovae became active and left the host cells. In such preparations, the merozoites flexed and glided more frequently and moved for longer distances than normal; in E. callospermophili, merozoites were observed penetrating new host cells and sporozoite-shaped schizonts were seen leaving host cells. Speer, Hammond, and Anderson (1970) reported that Eimeria callospermophili merozoites which developed in cell cultures underwent a marked increase in motility when treated with a trypsin-bovine bile solution. We report herein additional information concerning stimulation of motility in merozoites of E. callospermophili, E. larimerensis, and E. bilamellata from the Uinta ground squirrel, E. nieschulzi from the rat, and E. ninakohlyakimovae from sheep. All of the above species of coccidia were grown in cell cultures. MATERIALS AND METHODS Cell lines of embryonic bovine intestine, kidney, and liver were used in this investigation. The methods for maintaining and examining the cell monolayers were similar to those described by Fayer and Hammond (1967). Oocysts were collected and handled as in previous work (Speer, Hammond, and Anderson, 1970; Kelley and Hammond, 1970). Monolayers from Leighton tube cultures were examined in double-coverslip preparations (Parker, 1961) with phase-contrast microscopy at the appropriate interval after inoculation of sporozoites of each Eimeria species at which mature schizonts usually occur (24 hr after inoculation for E. callospermophili, 36 hr for E. nieschulzi, 48 hr for E. larimerensis, 4 days for E. bilamellata, and 10 days for E. ninakohlyakimovae). Extracellular merozoites or mature first-generation Received for publication 17 February 1970. Supported in part by research grant AI-07488 from the NIAID, U. S. Public Health Service. Published as Journal Paper No. 1005, Utah Agricultural Experiment Station. schizonts with merozoites were located and the degree of motility was observed before applying various solutions of trypsin and/or bile salts or bovine bile in minimal essential medium (MEM, Nutritional Biochemical) to the edge of the double cover slip. The solutions included 0.2% trypsin (1-300, Nutritional Biochemical) and 4% bovine bile, 0.2% trypsin and 0.5% sodium taurocholate (bile salt), 0.2% trypsin, 0.5% sodium taurocholate, 0.5% sodium glycotaurocholate, 4% bovine bile, and fresh MEM as a control. The pH of all solutions was adjusted to approximately 7.4 with a 7.5% sodium bicarbonate solution. During observation, the monolayers were maintained at 37 C with the aid of a warm stage and additional examinations in each experiment were conducted at room temperature (about 22 C).