Two groups of 32 laying hens and two groups of 32, 23-day-old broiler chickens were given 2.5 and 5 mg kg-1 of aflatoxin in their diet for four, eight, 16 and 32 days; 16 hens and 32 broiler chicks served as control groups. In the laying hens, aflatoxicosis was characterised mainly by an increase in prothrombin time, and decreases in total plasma proteins and albumin levels; there were increases in red blood cell (RBC) counts and haematocrit which were greater in the group fed 5 mg kg-1. There was a decrease in beta-globulins and an increase in gamma-globulins. In the broiler chickens, there was only a slight increase in prothrombin time and a decrease in RBC counts. This data proves the good sensitivity and utility of prothrombin time values as an indicator of aflatoxicosis in birds.