The object of this study is the heat production size spectra, i.e. the integral value of heat production over the size fractions (standard increments of logarithm of the particle size) of microplankton (organisms < 120 mu m), The particles of the sea water samples analyzed were separated and concentrated into fractions of different sizes by nylon nets, Sartorius membrane filters and a specially constructed fractionation-concentration unit. The heat production of the fractions was registered by a Thermal Activity Monitor during incubation times of 40-50 h at 20 degrees C in the dependence on the sample volume, the fraction composition, and the reference charge. 20 experiments were conducted between December 1993 and April 1994 in the Sevastopol Bay, a Black Sea harbour under anthropogenic influence. The results obtained reflect the good metabolic state of the concentrated organisms. The heat production size spectra were quite comparable with formerly obtained size spectra of the intracellular adenosine triphosphate content. The microcalorimetric signal level(10-30 mu W) evidenced the applicability of this method for studying organisms of the femto- and picoplankton range (<0.2 and 0.2-2.0 mu m, respectively) and for monitoring microbial communities of oligotrophic acid deep sea waters.