We have reported that the maximal velocity of shortening and myofibrillar adenosine triphosphatase (ATPase) activity of antigen-sensitized airway smooth muscle are higher than that of nonsensitized airway smooth muscle (Kong, S. K., R. P. C. Shiu, and N. L. Stephens. J. Appl. Physiol. 60: 92–94, 1986). To extend these studies, we attempted to determine whether the increased myofibrillar ATPase activity from sensitized airway smooth muscle was associated with either a change in distribution of two myosin heavy chain isozymes or an increase in myosin light chain phosphorylation. Myosin heavy chain isozymes from both control and sensitized airway smooth muscle were separated by 4% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Gels were analyzed by densitometry, which indicated that isozyme band pattern of sensitized airway smooth muscle was not different from that of the control. The maximal levels of phosphorylated myosin light chain from whole cell homogenates of sensitized and control tracheal smooth muscles were 0.65 +/- 0.029 (n = 6) and 0.40 +/- 0.025 mol Pi/mol light chain (n = 6), respectively. The degree of phosphorylation of myosin light chain of sensitized airway smooth muscle was significantly higher than that of the control (P less than 0.05). This study also indicated that increased myofibrillar ATPase activity in sensitized tracheal smooth muscle was correlated with phosphorylation of myosin light chain.