A quantitative coculture assay is described for determining susceptibility of HIV-1 isolates to zidovudine and sCD4 (178)-PE40 (a 60 kDa hybrid protein consisting of the gp120 binding region of CD4 linked to the translocation and ADP-ribosylation regions of Pseudomonas aeruginosa exotoxin). The assay was relatively simple to perform and gave highly reproducible results often within three to five days. IC50 values of zidovudine for HIV-1 strains isolated from two AIDS patients and an asymptomatic seropositive individual were in the range 0.001-0.002 mumol. Isolates obtained after six months of zidovudine treatment had zidovudine IC50 values of 0.01-0.5 mumol. All isolates were equally sensitive to sCD4 (178)-PE40 (IC50 1-5 micrograms/ml). HIV-1 activation in the chronically infected cell line U-1 was inhibited by sCD4 (178)-PE40 but not by zidovudine.