Abstract: An interlock mechanism is provided for use as an adjunct with a camera having a shutter and a lanyard operated film advancing and processing system which utilizes a film cassette contaning, in stacked relationship, a dark slide and a plurality of self-processable film units. Each film unit is adapted to be exposed and then processed in a consecutive sequence. The interlock mechanism functions to preserve the proper operating sequence and prevents the camera user from performing anomalous operations which could result in film losses due to the processing of non-exposed film units.