In this paper we show that human erythroleukaemia (K562) cells exhibited a significant inhibition of protein kinase C activity when cells were exposed to 40 micro M zidovudine in a time interval of 5-180 min., whereas prolonged treatment (24 hr) was uneffective. The addition of an excess of thymidine (125:1, mol:mol), in the cell suspension with or without zidovudine fully restored the protein kinase C activity. Interestingly, either in cell homogenates and in commercially purified rat brain protein kinase C, both zidovudine and its monophosphate derivative, caused inhibition that was higher than in intact cells. This inhibition reached a maximal value of 45% when zidovudine or zidovudine monophosphate were incubated with the pure commercial enzyme and in this case the addition of thymidine did not prevent the enzyme inhibition. The conclusions from these data are that either zidovudine or zidovudine monophosphate interact directly with the pure enzyme, causing inhibition, while in intact cells exposed to the drug, zidovudine monophosphate appears to be the main metabolite responsible for protein kinase C inhibition.