Neural primordia of chick embryos were cultured for three days and the behaviour of migrating neural crest cells studied. Somite cells were used as a comparison. Crest cells were actively multipolar with narrow projections which extended and retracted rapidly, contrasting to the gradual extension of somite-cell lamelleae. On losing cell contact, somite cells were also more directionally persistent. The rate of displacement of isolated crest cells was particularly low when calculated over a long time base. Both crest and somite cells were monolayered; contact paralysis occurred in somite cell collisions but was not ascertained for crest cells. However, crest cells in a population were far more directionally persistent than isolated cells. Contact duration between crest cells increased with time and they formed an open network. Eventually, retraction clumping occurred, initially and chiefly at the periphery of the crest outgrowth. Crest cells did not invade cultured embryonic mesenchymal or epithelial populations but endoderm underlapped them. No effects were observed on crest cells prior to direct contact. Substrate previously occupied by endoderm of ectoderm caused crest cells to flatten while substrate previously occupied by the neural tube caused them to round up and clump prematurely.