A 170 kDa polypeptide that has been partially purified from mung beans is retained by ADPagarose even in the absence of divalent cations when most non-myosin ATPases and kinases do not bind. Attempts to demonstrate a myosin-like ATPase activity were inconclusive, however, and the protein accounts at most for only a small part of the total K+ EDTA ATPase activity of mung bean extracts. All four monoclonal antibodies raised to the 170 kDa polypeptide react with rabbit skeletal muscle myosin and localize the 170 kDa polypeptide in mung bean root tip cells to the actin-containing phragmoplast and to sites dispersed throughout the cytoplasm which probably include some but not all actin cables. These 4 monoclonals and 3 commercially available antimyosin monoclonals all recognise rabbit skeletal myosin and 160-170 kDa proteins that are present in two other angiosperms tested. In addition, a 158 kDa protein of mung bean reacts with only one antibody and does not bind ADP-agarose. We conclude that strong but not yet conclusive evidence points to the 160-170 kDa proteins of angiosperms being a widely conserved form of myosin heavy chain.