Abstract: A sample of a biological tissue and/or fluid, particularly plant sap, may be taken by placing biological tissue such as a leaf between a layer which is adsorbent for detectable biological material (for example nitrocellulose) and a substrate and applying pressure to urge the adsorbent layer and the substrate towards each other, thereby causing detectable biological material to be adsorbed on the adsorbent layer. The invention thus provides a convenient method of obtaining, securing and transporting small samples of plant sap or other fluid or tissue to the laboratory for such purposes as analysis for plant viruses and other organisms.