Summary Muscle cell fusion can be triggered by Ca ++. The curve of fusion percentage vs. [Ca ++] is much steeper than the Langmuir isotherme. The mid point of this curve (representing 50 % fusion) can be shifted from 95 μM to 1100 μM Ca ++ by lowering the pH from 7.6 to 6.3. In the same pH-range the steepness of the curves decreases by a factor of 3. The curves are discussed in terms of a cooperative Ca ++-binding process.