Using five species of mammalian and avian cells, the authors succeeded in preparing Triton-treated culture cells that contract upon addition of MgATP. The contraction of these Triton cell models was inhibited by N-ethylmaleimide-modified myosin subfragment-1, a specific inhibitor of actin-myosin interaction. Triton cell models adhered more strongly to the substratum than glycerinated ones. Triton cell models of mouse 3T3 and human MRC-5 cells showed Ca2+-sensitive contraction. They required Ca2+ of 1 microM or more for the contraction. Other Triton cell models and all glycerinated cell models did not require Ca2+ for the contraction. The Ca2+-dependent contraction of 3T3 and MRC-5 cell models was inhibited by chlorpromazine, an inhibitor of calmodulin. The Ca2+-sensitivity of the contraction was lost by pretreatment of these cell models with adenosine 5'-O-(3-thiotriphosphate) in the presence of Ca2+. These results agree with a hypothesis that Ca2+-calmodulin-dependent phosphorylation of myosin light chain regulates actin-myosin interactions in non-muscle cells.