We have used fluorescence recovery after photobleaching to study the effect of muscle alpha-actinin on the structure of actin filaments in dilute solutions. Unexpectedly we found that alpha-actinin partitioned filaments into two types: those with a high mobility and those with low mobility. We have determined that the high mobility (smaller sized) population is too large to be simple monomeric actin:alpha-actinin complexes. Although it is known that cofilin encourages the transformation of alpha-actinin:actin gels into large meshworks of inter-digitating actin filament bundles (Maciver et al. 1991), we have found that the presence of cofilin also increases the cross-linking of actin filaments by alpha-actinin and hypothesize that this is due to cofilin's ability to alter the filament twist. This effectively makes more potential alpha-actinin binding sites per unit of actin filament. As expected from previous work, this effect was more marked at pH 6.5 than at pH 8.0. Both effects are likely to operate in cells to deny other actin-binding proteins access to binding these particular filaments and may explain how very different actin cytoskeletal structures may co-exist in the same cell at the same time.