The drugs usually used to treat PCP in AIDS patients (trimethoprim / sulfamethoxazole and pentamidine) have had to be discontinued in many patients because of severe adverse effects. Currently there are no proven alternatives to these drugs. TMTX was chosen for this trial because it has been found to be very active against the PCP organism in laboratory tests. In a preliminary trial, TMTX in combination with LCV has been effective against PCP with fewer and less severe adverse effects.