The effect of vincristine (VCR), cytoxan and actinomycin-D on platelet aggregation was studied in 30 children with solid tumors. 2 h after administration of these drugs, a statistically insignificant reduction in the primary wave of aggregation was observed with the exception of ADP-induced aggregation after VCR injection. Unobtainable secondary wave aggregation was seen in 28, 33 and 40% of the patients after administration of VCR, cytoxan and actinomycin-D, respectively. However, none of the patients had clinical bleeding. It was concluded that after the intravenous administration of therapeutic dosages of these drugs, the bleeding tendency caused by inhibition of aggregation is not to be expected.