Abstract: With the help of PCR it is possible to detect parasites of the genus Cryptosporidium by amplifying specific gene sequences. The disadvantage of this method is that it does not allow a statement about whether the detected gene sequences have been derived from viable and infectious parasites or from dead organisms or even from free DNA which has been released from organisms which have already disintegrated. However, by the combination of PCR with other known methods, i.e. an in vitro excystation assay and the removing or the destruction of nucleic acids which might be present in a sample material outside of Cryptosporidium oocysts before excystation this problem can be solved. Due to this combination the result of a PCR for Cryptosporidium allows a judgement on the infectivity of the examined material which would not be possible without this combination.