The locomotory invasive ability of HeLa cells was tested against: (a) embryonic chick heart fibroblasts (CHF); and (b) normal epithelial cells from human cervix (HCE) in explant confrontations. Data for analyses were obtained from replicate cultures fixed 24 h after junction and from 24-h time-lapse films. The mean invasion index for HeLa versus CHF did not indicate significant obstruction but analyses of hourly radial advance and orientation frequencies showed that obstruction eventually developed as postjunctional incubation time increased. Early contacts between HeLa and CHF demonstrated non-reciprocity of type I contact inhibition of locomotion by the tumour cells, which continued moving in their original direction to underlap contact-inhibited fibroblasts and eventually to occupy spaces vacated by them. When CHF population density increased and free space diminished, HeLa cells displayed directional and probably substrate-dependent contact inhibition. The high invasion index of HeLa versus HCE was largely due to occupation of previous HCE territory by tumour cells and only occasionally to actual infiltration of the epithelial sheet. After contact with HeLa, ruffling substrate-adherent marginal epithelial cells displayed contractile, type I contact inhibition of locomotion. After orientation changes, they gradually retreated. Against HCE, HeLa cells exhibited non-reciprocity of type I contact inhibition and continued radially forward, following the retreating epithelial margin. They did not move onto exposed upper surfaces of epithelial cells and did not underlap marginal cells firmly adherent to the substratum. Invasion of the epithelial sheet was seen only when initial access beneath a cell with a non-adherent margin was available. The contact relationships of isolated invading HeLa cells with their epithelial neighbours suggested successive non-reciprocal contact inhibition reactions.