In this paper we propose a real time computation algorithm based on the bufferless fluid flow model for call admission control in ATM networks with heterogeneous bursty traffic. Cell loss probability is adopted as the measure of quality of service. Our proposed computation algorithm requires a constant memory size and needs only two multiplications and one division to decide whether a connection request can be accepted or not. It is known  that the individual cell loss probability for each type of traffic sources may not meet the requirement even though the global one does. We provide in this paper a tight upper bound for individual cell loss probability which can be evaluated easily by our proposed computation algorithm. Numerical examples using typical traffic parameters are studied.