Abstract We have repeated Sanger's  experiments on the staining of dividing cells with fluorescent-heavy meromyosin (HMM), with some important modifications in technique, and confirmed some, but not all, of his observations. Most importantly, we confirmed that fluorescent-HMM is concentrated in the mitotic spindle, even in cells that are fixed with formalin prior to staining. The fluorescent-HMM used in these experiments was purified by ion exchange chromatography, a step which eliminates all detectable non-specific staining. Controls for the specificity of the fluorescent-HMM staining were competition with unlabeled HMM and chemical inhibition of actin-fluorescent-HMM interaction with Mg-pyrophosphate. Our fluorescent-HMM staining patterns differed from Sanger's in three ways. We observed no staining of kinetochores and no concentration of staining in the cleavage furrow, while we found that fluorescent-HMM was concentrated in the interzone during anaphase.