RNA-directed DNA polymerase (reverse transcriptase) activity was detected in platelets from 4/4 patients with primary proliferative polycythaemia (PPP), 7/7 patients with essential thrombocythaemia (ET), 1/4 patients with relative "stress" polycythaemia, 0/2 patients with secondary polycythaemia, and 0/3 normal subjects. The activity appeared to be particle-associated and was detected under conditions not appropriate for any known cellular enzymes. Active particulate fractions from 1 patient with PPP and 1 patient with ET were examined by electron microscopy and revealed objects with the features of retroviruses. Similar retrovirus-like particles were observed in 2/2 further patients with PPP and 2/2 further patients with ET but in 0/3 controls.