Adult male mice were treated with one or two different doses of each of 18 different cytotoxic agents. They were sampled at various times (3-12h) thereafter, and the spatial distributions of cell death in the small intestinal crypts were studied. Dead or dying cells or cells carrying dead cell fragments were examined histologically, and all of these were recorded (for convenience as apoptotic fragments), relative to the cell position in the crypt. Thus, distributions of apoptotic fragments against cell position were determined. A regression analysis of the data obtained at different times after administration of each agent was undertaken and the position of the median of the spatial distribution of presumptive target cells was deduced for each cytotoxic agent. The accuracy of this median value was determined to be +/- 0.5 cell positions. From these median values, the different cytotoxic agents could be divided roughly into three groups: [3H]thymidine, isopropyl-methane-sulphonate, gamma-rays, bleomycin and adriamycin all have their median values (susceptible cells) at cell positions 4 to 6; bischlorethylnitrosourea, actinomycin D, cyclophosphamide and cycloheximide at cell positions 6-8; mechlorethamine, triethylenethiophosphoramide, vincristine, 5-fluorouracil, hydroxyurea and methotrexate at cell positions 8-11. The position of these medians was considered in relation to the killing of clonogenic cells. Preliminary studies on the distributions of dead cells after myleran, cis-platinum and heat (hyperthermia) were also reported. There is a general tendency for antibiotics and radiation to attack the lower cell positions in the crypt. Alkylating agents on the other hand have a somewhat broad spectrum of action. Antimetabolites and a microtubule dissociating agent act on higher cell positions. No difference could be detected between two different forms (sources) of actinomycin D. The changes in the yields of apoptotic and mitotic cells with time and the migration velocities of cells in the crypts carrying apoptotic fragments after exposure to cytotoxics are also presented.