Compared with the vast amount of literature which exists on many aspects of latices, relatively little is known about the ionic composition of these fluids. Adriani (1850) mentioned the presence of large amounts of magnesium in the latex of Ficus elastica, Fickendey (1909) in the latex of Funtumia elastica. Molisch (1901) reported considerable amounts of magnesium, calcium and chloride in the latices of a large number of plant species. Ultee (1925), in an endeavour to make a classification of plant latices in general, distinguished the group of ’’salt-rich” latices from other groups such as the ’’rubber-rich”, the ”protein-rich”, and the ”phytosterin-rich” latices. With Hevea brasiliensis a considerable amount of work has been done on the mineral elements of the latex (Flint & Ramage 1935), especially in connection with latex stability (Beaufils 1954, Belmas 1952, Resing 1955). In the course of a study of latex particles of Hoya and Euphorbia species the possibility was investigated that the potassium and magnesium ions in the latex have something to do with the stability of the dispersed terpenoid particles present in it. Therefore a number of magnesium and potassium determinations have been carried out using standard atomic absorption determination methods. Latex expelled within 30-45 seconds following incision of the plant with a razor blade was collected with 50 [xl glass capillaries. EDTA and Sr(N03)2 were added prior to the Mg determination to eliminate a possible protein interference (Alcock & Mac-Intyre 1966). Values for K and Mg obtained from diluted latex and from latex ash were in agreement. The plant species used are all in cultivation in the greenhouses or in the botanic garden of the laboratory. As indicated in label 1 an uncertainty exists about the correct naming of some Hoya species. Most plants used are vegetatively propagated specimens, and therefore genetic diversity within the species could not be investigated more extensively. Tables 1 and 2 list the data obtained from some forty species. Table 3 shows that the latex composition, as far as these minerals are concerned, is of a reasonable constancy in clonal material of H. australis R. Br. Material of H. diversifolia was collected in the field (Java, Indonesia) at various locations; it gives some indication about the variation which may naturally exist within a species.