DAPI即4',6-二脒基-2-苯基吲哚(4',6-diamidino-2-phenylindole)，是一种能够与DNA强力结合的荧光染料，常用于萤光显微镜观测。因为DAPI可以透过完整的细胞膜，它可以用于活细胞和固定细胞的染色。

在萤光显微镜观察下，DAPI染剂是利用紫外光波长的光线激发。当DAPI与双股DNA结合时，最大吸收波长为358nm，最大发射波长为461nm，其发散光的波长范围含盖了蓝色至青绿色。DAPI也可以和RNA结合，但产生的萤光强度不及与DNA结合的结果，其发散光的波长范围约在400nm左右。

DAPI (4',6-diamidino-2-phenylindole) is a fluorescent stain that binds strongly to A-T rich regions in DNA. It is used extensively in fluorescence microscopy. As DAPI can pass through an intact cell membrane, it can be used to stain both live and fixed cells, though it passes through the membrane less efficiently in live cells and therefore the effectiveness of the stain is lower.