Herein we report that crotamine, a small lysine- and cysteine-rich protein from the venom of the South American rattlesnake, can rapidly penetrate into different cell types and mouse blastocysts in vitro. In vivo, crotamine strongly labels cells from mouse bone marrow and spleen and from peritoneal liquid, as shown by fluorescent confocal laser-scanning microscopy. Nuclear localization of crotamine was observed...