Chlorella mass cultures form a good experimental model for photobiochemical measurements as they contain small globular cells of 3–6 μm diameter in a well-mixed, homogeneous suspension. A linear relationship was found between optical density measured at 750 nm (OD₇₅₀) and biomass concentration, chlorophyll content and cell number within a range of biomass densities from 1 to 46 g L⁻¹, which represented between 30–1,400 mg L⁻¹ chlorophyll and 4–200 mg L⁻¹ carotenoids, respectively (Table 1; Fig. 4). Per gram of dry matter, the Chlorella cultures contained about 30 × 10¹² cells, ~28 mg chlorophyll a + b and ~4 mg total carotenoids. At the maximum daily irradiance, the biomass of the highly productive Chlorella culture (w/w in dry biomass) contained 3–3.5% total chlorophyll and 0.4–0.5% total carotenoids. The HPLC analysis showed that the Chlorella biomass contained mostly lutein (64%), violaxanthin (10%), neoxanthin (9%), β-carotene (10%) and some other carotenoids. The carotenoid-to-chlorophyll ratio usually increased at midday in cultures grown at concentration below 6 g L⁻¹. Only in diluted cultures, grown at high irradiance, were noticeable amounts of the xanthophyll-cycle pigment, zeaxanthin, detected (Table 1).