Chalo DM, Lukhoba C, Fidahussein DS, Nguta JM. 2017. Antimicrobial activity, toxicity and phytochemical screening of selected medicinal plants of Losho, Narok County, Kenya. Biofarmasi (Rumphius J Nat Prod Biochem) 15: 29-43. In Kenya, microbial infections are a major cause of morbidity. The effectiveness of antibiotic is threatened by the increase of resistance of pathogenic microbes against most available drugs because new pathogens continue to emerge. Nowadays, herbal remedies offer hope since they are readily available and cheap. The aim of this research was to investigate the activity of antimicrobial, the lethality of brine shrimp and phytochemical composition of crude extracts of four selected plants namely Schrebera alata (Oleaceae), Ormocarpum kirkii (Papilionoideae), Helichrysum forskahlii (Asteraceae) and Cussonia holstii (Araliaceae) that are medicinally used by herbalists from Losho, Narok County Kenya for treatment of ear, nose and throat infections, gastrointestinal disorders and skin diseases. Qualitative antimicrobial susceptibility test against five microorganisms, methicillin-resistant Staphylococcus aureus, Bacillus cereus, Escherichia coli, Pseudomonas aeruginosa and Candida albicans was investigated using agar diffusion methods to produce inhibition zones and the data accrued were analyzed using Analysis of variance. Minimum inhibitory concentrations were determined by broth microdilution method. Toxicity of the extracts was analyzed using brine shrimp lethality assay. The median fatal concentration of fifty was determined by data analysis using Finney‟s computer program. Phytochemical screening for flavonoids, sterols, alkaloids, tannins, quinones and terpenoids and saponins was determined using standard procedures. The observation showed that the organic crude extracts of H. forskahlii had the highest inhibition zone against methicillin-resistant S. aureus of 19.5 and 18.5 mm in agar well and agar disk diffusion respectively. In addition, organic extracts of H. forskahlii showed the highest antifungal inhibition zone of 8.5 mm in agar well diffusion. Minimum values of inhibitory concentrations varied from 15.625 to 250 mg/mL. Organic crude extracts of H. forskahlii and C. holstii were found to be highly toxic with lethal concentration of 0.009 mg/mL. All plant crude extracts contained flavonoids, sterols, alkaloids, tannins, quinones, and terpenoids. Saponins were present in all the plant extracts except in organic extracts of H. forskahlii. This study promoted the first record of antimicrobial activity, toxicity and phytochemical composition of S. alata and C. holstii. The study has shown that H. forskahlii and O. kirkii possess promising antimicrobial activity against microbes of health importance and could lead to the isolation of new, safe and efficacious antimicrobial compounds. Further research should be carried on O. kirkii and S. alata to isolate and characterize the compounds responsible for the observed activity.