In sequential diagnostic reasoning, observed evidence activates hypotheses about possible causes in memory. These memory activations have been previously examined with a probe reaction task for problems with a single correct diagnosis. We applied this process tracing method to ambiguous problems with multiple compatible hypotheses. When participants reasoned about the causes of ambiguous symptom sequences, they were prompted to respond to probes representing hypotheses. The response time to a probe was shorter if the current support for the respective hypothesis was stronger indicating that the processing of compatible hypotheses can be traced. For sequences with two equally supported hypotheses, the initial hypothesis was more often chosen as the final diagnosis (a primacy effect). Probe reaction times suggest that the initial hypothesis has been activated more strongly already early, when it was finally chosen as the diagnosis. Nevertheless, substantial variance in response times limits the task's applicability for process tracing.