A micropore membrane procedure to assay taxis by Entamoeba histolytica is described and the results of studies of responses to a variety of soluble substances, bacteria, an rat colon washings using this procedure are reported. Trophozoites migrated in blind well chambers through 8-micron pore size polycarbonate membranes but not nitrocellulose membranes up to 12 micron pore size. Amoebae were attracted toward fresh axenic culture medium (TYI-S), an enzymatic hydrolysate of casein (Trypticase), and a partially purified preparation of N-acetylneuraminic acid from egg mucin, but not purified N-acetylneuraminate or a variety of other low molecular weight metabolites. The response was verified as chemotaxis by checkerboard analysis. Amoebae migrated most dramatically toward suspensions of all of seven bacterial species tested, including motile and non-motile, gram-negative and gram-positive rods and cocci. This response was diminished when the bacteria concentration gradient was eliminated. The response to bacteria culture filtrates was less than 10% of that to bacterial suspensions. A response to clarified washings from the rat colon was detected; this was diminished but not eliminated by filter sterilization of the washings. We concluded that some soluble molecules, possibly of intermediate molecular size, whole bacteria, and both soluble and particulate components of the rat colon provide tactic stimuli for E. histolytica. Scanning electron micrographs of trophozoites migrating towards attractants through membranes showed narrow, extended pseudopodia entering the membrane pores, and enlarging spheres exiting as the cells proceeded through.