We propose an adaptive load balancing algorithm for heterogeneous distributed systems. The algorithm intrinsically allows a batch of tasks to be relocated. The key of the algorithm is to transfer a suitable amount of processing demand from senders to receivers. This amount is determined dynamically during sender-receiver negotiations. Factors considered when this amount is determined include processing speeds of different nodes, the current load state of both sender and receiver, and the processing demands of tasks eligible for relocation. Composition of a task batch is modeled as a 0-1 Knapsack problem. We also propose a load state measurement scheme which is designed particularly for heterogeneous systems.