A photosynthesizing microorganism culture, like Spirulina platensis, can be an alternative source of protein for human food and feed purposes, with the possibility of obtaining other products like pigments (carotenoids, phycocyanin and chlorophyll), vitamins and lipids. The biomass obtained can be introduced directly in the diet and it can also be used in cases of malnutrition. This type of biomass has been used as a food since ancient times in some regions of Africa and Mexico, from where Spirulina sp. originated. In this work, the influence of inoculum age in S. platensis cultivation in Erlenmeyer's flasks and in mini-tanks was studied. After the determination of the best inoculum age, a study of the influence of inoculum concentration in mini-tanks cultivation was performed. Assays with four different inocula ages: 3, 6, 10 and 14 days and four different concentrations: 50, 100, 150 and 200 mg l(-1) were made. The biomass obtained was analyzed in terms of their chemical composition and chlorophyll contents. The results showed that the best inoculum age was six days with 50 mg l(-1) of concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.