Among the haemorheological parameters, red blood cell (RBC) aggregation shows the largest interspecies diversity, and often controversial data can be found in the literature, besides the methodology-dependent issues. In this present investigation, we compared four experimental/laboratory animal species' RBC aggregation by two different photometric methods for better revealing the differences. Blood samples (K3-EDTA, 1.5 mg/ml) were taken from female animals: 16 inbred mice (Mus musculus, cardiac puncture), 15 outbred rats (Rattus norvegicus, caudal caval vein puncture), 15 beagle dogs (Canis canis, cephalic vein) and 23 juvenile pigs (Sus scrofa domesticus, medial saphenous vein). Haematological parameters (microcell counter) and RBC aggregation (light transmission and syllectometry-laser backscatter methods) were determined within 2 h after sampling. Describing the first 5–10 s of the aggregation process, additional parameters were calculated out of the syllectometric raw data. Standardized difference was calculated to determine the sensitivity of the two devices. Parameters describing the extent and magnitude of red blood cell aggregation showed the lowest values in the rat and the highest in the pig and canine blood. In turn, parameters describing the kinetics of aggregation showed the lowest values in the mouse and the highest in the rat. The standardized difference values for the laser backscattering method were 2–4 times larger vs. the light transmission one. The magnitude of the differences was not consequent in the aggregation parameters. These comparative results show that the laser backscattering method can detect the RBC aggregation differences between the investigated species more sensitively than the light transmission method.