The latex of two plants and the methanol and petroleum ether extracts of thirty four plants from twenty one families were screened for their biological activity using the brine shrimp lethality test. The plants were selected on the basis of their traditional medicinal uses in Kenya. Thirty six of the seventy eight samples showed toxicity to the brine shrimp (LCso < 1000 ug/ml). The most active extracts (LCso < 250 Ilg/ml) were from Bridelia micrantha, Brucea antidysenterica, Croton megalocarpus, Engleromyces goetzi, Euclea schimperi, Mondia whitei, Persea americana, Phytolacca dodecandra, Salvia nilotica, Tagetes min uta, Teclea simplifolia and Toddalia asiatica. The results could be useful in the search for new compounds with pesticidal, antitumour and other pharmacological actions.