This paper is a review of previous work with heavy meromyosin (HMM) binding and describes some new findings. The HMM technique can be used to effectively determine changes in the organizational states of actin like microfilaments which accompany various motile activities. The elucidation of such changes is central to an understanding of how microfilaments function in cell motility. Emphasis is also placed on the interpretation of ultrastructure following glycerol extraction and HMM binding. The overall evidence presented demonstrates that in situ HMM binding can be used as an effective ultrastructural cytochemical probe for determining the distribution and possible functions of actin like microfilaments in nonmuscle cells. Since actin like proteins appear to represent a large proportion of the total cell types, their ultrastructural localization and organization is central to an understanding of how actin functions in cell motility. Further understanding of the dynamic nature and interconversion of the various organizational states of actin like microfilaments will ultimately help to explain many aspects of cell surface movement including membrane ruffling, pinocytosis, cytokinesis, contact inhibition, membrane fluidity and perhaps the topographic distribution of components on the outer surface of the plasma membrane.