The ORNL Analytical Chemistry Division has developed a partially automated procedure for assay of transfer ribonucleic acids that utilizes the reaction between a tRNA and a14C-tagged amino acid, followed by precipitation and counting of the reaction product on a filter-paper disk. In this procedure, a Technicon AutoAnalyzer is used to take tRNA samples, mix them with the amino acid and suitable buffers and enzymes, and incubate the mixture. After incubation, segments of ca. 56-[I volume are transferred from the AutoAnalyzer output stream to small filter-paper disks by an output sampler of our design. This is done by alternately pressing the tip of the output tube against a paper disk for a measured time, and against the orifice of a waste aspirator. The output sampler is synchronized with the Technicon input sampler by using a dye in the rinsing solution that is picked up between samples at the input; barren zones of the product stream are colored, and sample-rich zones remain nearly colorless. The color actuates a photometric meter relay, whose operating point may be set for either the rise or fall of color in the stream. Once the relay has closed, it will remain so until one sample sequence is completed: an initial delay of 60 sec (or less) may be introduced to ensure that the sampling period spans a peak input-sample concentration. After the delay, a cam driven by a synchronous motor moves the output tip from the waste aspirator into contact with apaper disk. As this cam completes its revolution, it switches a second cam-driving